global water pathogen project part four. management … · sludge management is an integral...

GLOBAL WATER PATHOGEN PROJECTPART FOUR. MANAGEMENT OF RISK FROM EXCRETA AND WASTEWATER

SLUDGE MANAGEMENT:BIOSOLIDS AND FECAL SLUDGE

James MihelcicUniversity of South FloridaTampa, United States

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Citation:Mihelcic, J.R. 2018. Sludge Management: Biosolids and Fecal Sludge. In: J.B. Rose and B. Jiménez-Cisneros,(eds) Global Water Pathogen Project. http://www.waterpathogens.org (J.R. Mihelcic and M.E. Verbyla) (eds)P a r t 4 M a n a g e m e n t O f R i s k f r o m E x c r e t a a n d W a s t e w a t e r )http://www.waterpathogens.org/book/sludge-management Michigan State University, E. Lansing, MI,UNESCO.https://doi.org/10.14321/waterpathogens.69Acknowledgements: K.R.L. Young, Project Design editor; Website Design: Agroknow(http://www.agroknow.com)

Last published: August 30, 2018

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Sludge Management: Biosolids and Fecal Sludge

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Summary

Sludge, biosolids, and faecal sludge are generatedduring management of wastewater and the contents oflatrines. The direct disposal of untreated sludge is notdesirable because it: 1) has odors, 2) is comprised primarilyof water which makes transport and disposal expensive,and 3) contains harmful environmental pollutants andpathogens. Several processes can decrease pathogenconcentrations in sludge: 1) stabilization, 2) thickening, 3)dewatering, and 4) other processes that includecomposting. Stabilization uses biological, chemical, and/orthermal processes to reduce organic matter, water content,and odors and also provides some pathogen reduction.Thickening and dewatering can reduce volume andincrease the concentration of total solids. Thickening isperformed prior to stabilization while dewatering is thefinal method of volume reduction before ultimate disposalof stabilized sludge (drying beds and mechanicalprocesses). Other methods of sludge processing includingcomposting, heat drying, and combustion.

Temperature and storage time are the two mostimportant factors in achieving significant pathogendestruction. Moisture content and pH are also important.Examples of sludge management technologies that canachieve temperatures greater than 45ºC if properlydesigned and operated are: 1) composting, 2) thermaltreatment, and 3) thermophilic digestion. For lowertemperature treatments, pathogen concentrations arereduced but still routinely identified in treated sludge.Other ways to improve pathogen destruction are reducingmoisture content by drying (through evaporation,transpiration by use of deep rooted plants, and drainage ofwater vertically from a drying bed), raising pH, and/orincreasing the concentration of unionized ammonia (NH3).Sunlight’s role is mostly through supporting desiccation ofdrying sludge. Biological processes (e.g., digestion,composting) can reduce the volatile organic fraction ofsludge; thus, reducing disease vector attraction. Helmintheggs are particularly resistant to treatment, includingthermal and chemical treatment. Therefore, in regions

where helminth infections regularly occur, it isrecommended to first provide treatment in sludge-dryingbeds, and then to bury the dried sludge to minimizeexposure. The World Health Organization also providesrecommendations for storage treatment of dried excretaand fecal sludge. They recommend an additional barrier ofsafety of waiting one month between the time storedexcreta is applied to a crop and the time the crop isharvested.

Faecal sludge is a distinct type of sludge that isgenerated onsite (e.g., discharge to toilet/latrine pits andseptic tanks) and is not transported through a sanitarysewer. It has different physical-chemical characteristicsthan sludge generated at a centralized treatment plant. Theservice chain for faecal sludge management shows apotential for increased human health risk associated withpathogen exposure during the collection of the material andsubsequent transport to a treatment or disposal facility.Thus design and management of toilet/latrine and septicsystems must consider safe collection and transport of thecontents.

1.0 Brief Technology Description

Sludge is generated during primary and secondarytreatment of wastewater. “Biosolids” is a word used todescribe sludge that receives treatment at a centralizedfacility and meets land application standards (NRC, 2002).“Fecal sludge” is a term used to describe sludge generatedfrom an onsite sanitation technology that has not beentransported in a sanitary sewer. It can be “raw or partiallydigested, a slurry or semisolid, and results from thecollection, storage or treatment of combinations of excretaand blackwater, with or without greywater.” (Strande et al.,2014). The fundamental mechanisms of how pathogenlevels are reduced in the management of sludge, biosolids,and fecal sludge are similar as are the treatmenttechnologies. Accordingly, for the remainder of thischapter, the word sludge will be used to describe bothbiosolids and fecal sludge, except in locations wheremanagement strategies for a particular sludge type maydiffer.


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The direct disposal of untreated sludge is not desirable because: 1) it has odors associated with it, 2) it is comprised primarily ofwater which makes transport and disposal more expensive, and 3) it contains harmful environmental pollutants and pathogens(Mihelcic and Zimmerman, 2014). An important point to consider is that in a centralized wastewater treatment plant, while the totalvolume of sludge is small compared to the total volume of wastewater, sludge management can cost up to 60 percent of the overalloperating costs of wastewater treatment (Andreoli et al., 2007). Also, if managed incorrectly and released into the environmentwithout adequate treatment, conventional pollutants and pathogens found in sludge can negate the health and environmental benefitsof wastewater treatment. Sludge management is an integral component of centralized or semi-centralized wastewater systems (Figure1) and onsite systems (shown later in Figure 6), serving cities and small communities alike. Fecal sludge management has alsobecome a distinct area of sludge management with books and conferences devoted to it (e.g., Strande et al., 2014).

Figure 1. Location of sludge management within the sanitation service chain for a centralized or semi-centralizedtreatment facility (a figure of the service chain for fecal sludge is provided in Section 5)

There are many technologies to decrease pathogenconcentrations in wastewater sludge. These technologiescan be broken down into the following processes: 1)stabilization 2) thickening, 3) dewatering, and 4) otherprocesses that includes composting. Examples oftechnologies associated with these processes aresummarized in Table 1. Sludge stabilization typically uses

biological and chemical processes to reduce odor, watercontent, and the presence of pathogenic organisms insludge. The purpose of dewatering is to reduce watercontent because sludge associated with primary andsecondary treatment of wastewater only has a solidscontent of half to several percent (sludge is thus mostlycomprised of water in its original untreated state).


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Sludge Treatment Process Objective Example Technologies

Stabilization

to reduce problemsassociated with sludgeodor and putrescence

and the presence ofpathogenicorganisms. Ituses biological,chemical,

and/orthermal

processes toreduceorganic

matter, watercontent, and

odors andalso provides

somepathogenreduction.

Biological Digestion (aerobic or anaerobic)

Chemical (e.g., alkaline (lime)) stabilization

Table 1. Processes (and example technologies) to treat wastewater sludge and faecal sludge

Lagoons

Reed beds

Drying beds

Centrifugation

Mechanical belt presses

Thickening

increase inresult in an

they may alsopathogens,number ofof the total

the reductionmay result in

processesdewatering

Thoughprocesses).mechanical

and(drying beds

sludgestabilizeddisposal ofultimatebefore

reductionvolume

method ofis the finalDewatering

whilestabilization

prior toperformed

Thickening issolids. of total

concentrationincrease thevolume and

sludgereduces the

sludge whichdewaterused to

Thickening and Dewatering

volume.overall

reduction ofdue to the

of pathogens,concentration

volumetricthe


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Sludge Treatment Process Objective Example Technologies

Other Methods

used todewater

sludge, whichreduces the

sludgevolume andincrease the

concentrationof total solids

Composting

Heat Drying

Combustion

For more information about sludge treatment anddisposal or reuse, refer to Andreoli et al. (2007), Drechselet al. (2010), Burton et al. (2014), Oakley and Salguero(2011), Oakley et al. (2012) and Semiyagaa et al. (2015).Strande et al. (2014) provide a comprehensive book onfecal sludge management. Several journal articles (e.g.,

reduction of pathogens.chemicals, and pathogens. This chapter is only focused onother undesirable components: metals, trace organictreatment. Besides odorous compounds, sludge may havewastewater treatment during centralized and onsitesludge generated during primary and secondary

Table 2 provides information on the typical mass of

).et al., 2014Bruuncreating and releasing methane into the atmosphere (

biological stabilization processes are not just simplymass basis), care needs to be taken to ensure thatpowerful greenhouse-forcing gas than carbon dioxide (on ageneration of electricity. Because methane is a much moreused for cooking and heating, or in large scale applications,and processed safely can provide a local source of energyapproximately 2/3 methane by volume, which if collectedgenerated during anaerobic stabilization consists ofnutrients are present in human urine, and not feces. Biogasphosphorus. This is because the majority of these twoit will contain less of the nutrients, nitrogen andseparate urine is still valuable as a soil amendment, though

). Fecal sludge obtained from processes that first2014

Dienera et al.,and using sludge as a soil conditioner (incorporating sludge as a component in building materials,protein from sludge processing to use as animal feed,generating biogas from anaerobic digestion, derivinguses included drying the sludge as a fuel for combustion,Saharan cities (Dakar and Kampala) and concluded feasibleidentified potential markets for fecal sludge in two Sub-pathogens are reduced to safe levels). A recent studyrecovery strategy (e.g., a viable soil amendment iffecal sludge are considered an integral part of a resourceenergy, carbon, nitrogen, and phosphorus), biosolids andBecause treated sludge contains valuable resources (e.g.,sludge and may also include biogas and other resources.

The outputs from sludge management include treated

2.0 Inputs and Outputs

.1983), Feachem et al. (1981associated with sludge management is available inprocesses. Historical information on pathogen fatepresent in sludge and their fate by various treatment

) provide literature reviews of pathogensPepper et al., 2010;Arthurson, 2008; Jiménez, 2007; Dumontet et al., 1999


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Table 2. Sludge generated during specific wastewater treatment processes

kg Suspended Solids

Sludge Produced / kg CODApplied

Percent Dry Solidsin Sludge (%)

Liters of Sludge Produced/ Person-day

Conventional Primary Treatmenta 0.35 to 0.45 2 to 6 0.6 to 2.2Septic Tank Primary Treatmenta 0.20 to 0.30 3 to 6 0.3 to 1.0Secondary (i.e., biological) wastewatertreatmenta 0.07 to 0.35 0.5 to 4.0 0.05 to 6.0

Ventilated Improved Pit (VIP) Toilet NR 17 to 23b 0.07 to 0.40c

Urine Diverting Toilet NR 40b 0.07 to 0.40d

NR: Not Reported

aAndreoli et al., 2007; bStrande et al., 2014; c filling rate reported by Foxan et al. (2011). Franceys et al. (1992)reported solids accumulation rates of 0.11 to 0.25 L/person-day; dassumed similar to VIP toilet

Table 3 provides expected concentration ranges of somepathogens present in specific types of wastewater sludge.In one review, Dumontet (1999) reported the followingmagnitudes of viral and bacterial pathogen concentrationsper kg in untreated sludge: 103 (viruses), 106 (Salmonella

spp.), 107 (Shigella spp.), 106 (Yersinia spp.), 103 (Brucellaspp.), and 105 (Staphylococcus coagulase positive). Themagnitude of the concentrations of specific pathogens inwastewater and sludge is however a function of social-economic factors that are specific to a particularcommunity and its health.

Table 3. Expected range of pathogen concentration in sludge (values obtained from Andreoli et al., 2007)

Pathogen Type of Sludge Typical Concentrations(Number per kg Total Solids)

Bacteria Sludge E+01 to E+07

E+01 to E+05Primary and digested sludgeVirusesE+01 to E+02E+04 to E+06E+04 to E+06

Dewatered sludgeDigested sludgePrimary sludge

Protozoan (oo)cysts

E+03 to E+04E+02 to E+05E+01 to E+03E+02 to E+03E+03 to E+04

Anaerobic sludgetreatment

Partially-dewatered sludge from aerobicPartially-dewatered sludge

Digested sludgePrimary sludge

Helminth eggs

http://www.waterpathogens.org/10281


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3.0 Factors Affecting Pathogens in Sludge Management Systems

During sludge processing, pathogens are inactivated by different mechanisms. These mechanisms will be similar for biosolids andfecal sludge. Figure 2 reviews the major mechanisms that lead to pathogen destruction in four common sludge managementtechnologies. The efficiency of each of these mechanisms is dependent on a number of environmental, design, and operational factors.Temperature and time have been identified as perhaps the two most important factors in achieving significant pathogen destructionduring sludge management (Sahlström, 2003). This is shown in the four figures presented in Figure 2.

Figure 2. Major factors affecting pathogens in sludge digesters (top left), sludge drying beds (upper right), compostingsystems (bottom left), and land application systems (bottom right). Photo of composting system © 2015 Chesapeake BayProgram (CC BY-NC 2.0). Photo of sludge land application system © City of Geneva (CC BY-NC 2.0)

Other mechanisms used to promote pathogendestruction during sludge treatment are desiccation (i.e.,reducing moisture content by air or heat drying), raisingpH, and increasing free (unionized) ammonia (NH3)c o n c e n t r a t i o n . B e c a u s e t h e p K a f o r t h eammonium/ammonia acid base reaction is 9.3, producingunionized ammonia can be achieved by raising the pH,which converts ionized ammonium (NH4

+) to NH3. Addition

of urine and urea have also been proposed as a source ofunionized ammonia (Vinnerås, 2007; Trimmer et al., 2016).Biological stabilization processes (e.g., digestion,composting) can reduce the volatile organic fraction ofsludge; thus, reducing disease vector attraction andbacterial growth because of loss of available organicsubstrate (USEPA, 2003). Sunlight’s role in decreasingpathogen concentrations in sludge is most likely through

).USEPA, 2003supporting desiccation of drying sludge (


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Table 4. Summary of factors and mechanisms for pathogen reduction in sludge

Mechanisms and Factors Evidence of Pathogen Vulnerability

Temperature/Time/Moisture

Inactivation of pathogens in sludge isa function of temperature, time, and

moisture content. The rate ofinactivation increases with increasingtemperature and decreasing moisture

content.

For instance, treatment at lowertemperatures requires more time at

the same moisture content to achievethe same level of pathogen reduction.

Long-term storage of dewatered sludge caninactivate all pathogens at the following

temperatures (WHO, 2006):2 to 20°C: 1.5 to 2 years

• Reduces all pathogens to low levels20 to 35°C: > 1 year

• Substantial to total inactivation of viruses,bacteria, and protozoa.

• More or less complete inactivation ofhelminths after 1 year.

Viruses• Poliovirus has been inactivated 3.0 log10 in

laboratory thermophilic digesters (49 to 55 °C)(Aitken et al., 2005).

Protozoa• High temperatures cause more rapid

inactivation of Cryptosporidium (oo)cysts(Reinoso et al., 2008)

Helminths• Temperature is the most important factor for

the inactivation of helminth eggs inthermophilic anaerobic digesters. A 3.0 log10

removal of Ascaris was documented attemperatures of 53 and 55°C (Aitken et al.,

2005).• Ascaris in composted excreta has been

inactivated in a solar thermal post-treatmentunit at the following levels (Darimani, 2015):2.0 log10: 2 hours at 50°C and 50% moisture3.0 log10: 3 hours at 60°C and 50% moisture

Alkaline stabilization withquicklime (CaO)

Application of CaO to dewateredsludge to raise the pH >12 causes an

exothermic reaction withtemperatures exceeding 50°C

Alkaline stabilization, if properly engineeredand operated, can inactivate viral, bacterial,

protozoan and helminthic pathogens (Burton etal., 2014).

Table 5. E. coli removal in mesophilic anaerobic digesters (summarized by Horan et al., 2004)

Operating Conditions Removal of E. coli (log10 units) Reference14 days retention time,10°C 0.5 Le et al., 2002

14 days retention time,25°C 1.0 Le et al., 2002

Primary sludgedigestion in full-scaleplants in England

1.0 Le et al., 2000

Humphrey, 19991.35 to 3.36Englandsludge facilities inSurvey of primary

Gantzer et al. 20011.5HRT in France35 to 37°C for 20 days

Le et al., 20001.5plants in Englanddigestion in full-scaleSecondary sludge





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Table 6. Time needed to reach 99% (T99) inactivation of Ascaris suum ova during anaerobic digestion of sludgeunder various temperatures (from Manser et al., 2016)

Digestion Temperature (°C) T99 Reference

10>

2,555days

Sanguinettiet al.

(2005)

20 450days

Pecson etal. (2007)

30 180days

Pecson etal. (2007)

35 22days

Manser etal. (2015)

40 5days

Pecson etal. (2007)

50110minto 4days

Pecson etal. (2007);Popat et al.

(2010)

The biological and chemical treatment of sludge doesnot necessarily inactivate all pathogens. Helminth eggs areparticularly resistant to sludge treatment processes. Forexample, Gaspard et al. (1995) found that helminth eggs intreated sludge from wastewater treatment plants in Francecontained the following percentages of viable eggs: 1)anaerobic digestion: 66%, 2) composting: 25%, and 3) limetreatment: 66%. In addition, Sahlstrom et al. (2003) foundthe following percentages of Salmonella still remaining intreated sludge from eight wastewater treatment plants inSweden: 1) mesophilic anaerobic digestion: 58%, 2)thermophilic anaerobic digestion: 12.5%, and 3)composting: 12.5%.

4.0 Design, Operation and MaintenanceGuidelines for Pathogen Reduction in Sludge

Management Systems

Of the four sludge stabilization technologies discussedin this chapter, composting of sludge after biologicalstabilization (e.g., anaerobic digestion) and heatpelletization (30-37°C for 10-20 days following by lowpressure oxidation drying systems) have been found toprovide the lowest levels of culturable fecal indicators andpathogen genomes. These two technologies were followedby temperature-controlled anaerobic digestion and finallymesophilic anaerobic digestion (Viau and Peccia, 2009).Thermophilic biological processes such as aerobic digestionand composting raise the temperature of the sludge whilebiological processes like anaerobic digestion may have heatadded to achieve thermophilic temperatures (sometimes byburning the methane found in the biogas output). Sludgemay also be disposed of in a sanitary landfill. While disposalmay be a safe management strategy, it limits opportunities



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for resource recovery.

4.1 Aerobic and Anaerobic Sludge Digestion

Digestion is a common method of sludge stabilization. Itis employed by large treatment plants, households, andsmall to large farms. During digestion, the organic andodorous fraction of the sludge is stabi l ized bymicroorganisms that degrade the volatile organic fractionof the sludge in the presence or absence of oxygen.Digesters can be fed continuously or intermittently. Lowrate (i.e., passive) digesters are typically mixed andoperated at ambient temperature while high rate (i.e.,active) digesters are mixed and may be aerated and/orheated. Sludge derived from a digester may be furtherprocessed by other treatment methods discussed in thischapter (e.g., dewatering technologies).

An important point of digester design and operation forpathogen reduction is that digesters can be operated undermesophilic (temperature range of ≈ 25-40°C) orthermophilic (temperature range of > 40°C) conditions. Ingeneral, because of its higher temperature, thermophilicdigestion is more efficient at reducing pathogen levels thanmesophilic digestion. It is in fact recommended that foranaerobic digesters operated in the temperature range of35-55°C, the solids retention time (referred to as SRT) ofthe sludge be at least 15 days while for 20°C the solidsretention time would need to increase to 60 days (USEPA,1994). To provide an additional general point on theimportance of temperature in pathogen removal, the t90 (theamount of time it takes for 90% or 1 log10 reduction) forbacteria ranges from hours to days for thermophilicdigestion and from weeks to months for mesophilicdigestion (Sahlström, 2003). When operated with the sameretention times, batch operations generally result in betterpathogen removal than continuous flow operations, due tothe fact that continuous flow operations result in partially-mixed conditions with short-circuiting that may cause someof the pathogens to remain in the reactor for much lessthan the average retention time (Manser et al., 2015).

The literature is not clear if the amount of volatile fattyacids (VFA) present in sludge has an impact on the fate ofpathogens. For example, VFAs were shown to inactivateShigella by more than 6.5 log10 units in night soil with 9,000mg/L VFAs after 18 days at pH 6.5 (Kunte et al., 2000) andSalmonella by more than 7 log10 units in an anaerobicsludge digester with 5,000 mg/L VFAs after 12 days at pH 6(Kunte et al., 1998). Ascaris eggs were completelyinactivated at pH 5 – 6 when exposed to VFAs for only 20hours in distilled water, but in sewage sludge, 82% of theeggs still remained viable after a week (Butkus et al.,2011). Typical reductions in the concentrations of fecalindicator organisms during digestion are reported to beapproximately 1-2 log10 units (Farrell et al., 1988).

4.1.1 Aerobic digestion

In aerobic digestion, a batch or continuous flow systemis set up that provides air to the sludge via mixing or someform of mechanical aeration. These reactors are typically

not heated but thermophilic bacteria that degrade organiccarbon during the process produce heat. A batch operatedaerobic digester can require aeration for up to 2-3 weeks.Aerobic digestion performed at a solids retention time of 40days and temperature of 20°C or a solids retention time of60 days and lower temperature of 15°C is expected toreduce bacterial and viral pathogens by 90% (USEPA,2003). As might be expected, thermophilic aerobicdigestion has much better removal of pathogenic bacteria,viruses, and parasites than mesophilic anaerobic digestion(Kabrick and Jewell, 1982).

4.1.2 Anaerobic digestion

Anaerobic digestion is a more common type of sludgedigestion. It takes place in an enclosed reactor (that may ormay not be heated). In an anaerobic environment void ofoxygen, organic material is reduced to carbon dioxide(approximately 1/3 of the gas volume), methane(approximately 2/3 of the gas volume), and ammonia. Thethree-step biochemical process includes hydrolysis,acidogenesis and acetogenesis, and methanogenesis. In alow-rate system, mixing is typically caused by gas bubblesthat rise through the sludge and influent sludge is addedintermittently. A high-rate system employs active mixingand controlled temperatures to increase the rate of sludgestabilization and pathogen inactivation (USEPA, 2003).

While a 1 to 2 log10 reduction has been observed forfecal coliforms and heterotrophic bacteria during anaerobicdigestion (Chauret et al., 1999), Clostridium perfringens,Enterococcus sp., and Giardia cysts are reported to have nostatistically significant reduction during anaerobicdigestion at 36°C with a retention time of 20 days. (Soareset al., 1994; Sykora et al., 1991; Chauret et al., 1999).Chauret et al. (1999) also reports the reduction of 0.09 log10

units of somatic coliphages during anaerobic digestion.

Infectious virus particles are reduced approximately 1log10 during full-scale mesophilic anaerobic digestion (36°C)in a primary digester operated at a hydraulic residencetime of 14 days followed by a secondary digester thatincreased the contact time by an additional 9 days. Pepperet al. (2010) reported that over a 17-year period study, theperformance of eighteen full-scale mesophilic anaerobicdigesters in the U.S. (serving populations of > 0.5 million)the average removal of enteric viruses was 94.4%,Salmonella was 97.6%, fecal coliforms was 98.9%, and totalcoliforms was 98.8%. If the temperature is raised to 50°Cvirus inactivation is reported to reach 99.99% (Sanders etal., 1979).

4.1.3 Small scale anaerobic digesters

Small scale anaerobic digesters can be constructedfrom brick or plastic materials (Figure 3). They are themost common anaerobic digesters used by farms andhouseholds in low-income countries. They are typicallyoperated under ambient conditions and fed on anintermittent basis with agricultural wastes (e.g., manure),which can be mixed with human waste. Because these


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systems produce methane (which has ≈25 times the globalwarming potential of carbon dioxide), before these systemsare implemented, it should be ensured that the resultingdemand for biogas will be ≥ gas production and systemsare regularly maintained to ensure no leakage of biogas tothe atmosphere.

Figure 3. Left) Filling a tubular anaerobic digesterwith sludge. Right) Tubular digester (dark cylindricaltube) located inside a clear plastic covering topromote higher temperature (photos provided byDanielle DeVuyst with permission)

However, use of anaerobic digesters operated in thismanner may not be sufficient to safely inactivate persistenthuman pathogens such as Ascaris spp. Because thesedigesters are typically fed intermittently, a user only hascontrol over the solids retention time and feeding interval.It is expected that these reactors also operate at lowertemperatures which may be near ambient. For example,one study observed the temperature of an operating tubularbiodigester in Costa Rica that was fed swine manure to benear ambient temperature (21-24°C) (Kinyua et al., 2016).

Kinyua et al. (2016) also concluded that temperatureand volatile fatty acid concentration were the main factorscontributing to Cryptosporidium parvum and Giardialamblia inactivation in tubular digesters. They reported a1.56 (± 0.24) log10 reduction for G. lamblia and 0.55 (±0.01) log10 reduction for C. parvum over a 24-day period ata temperature of 21°C and volatile fatty acid concentrationof approximately 300 mg COD/L.

Manser et al. (2016) developed a mathematical model toestimate the inactivation of Ascaris suum ova in household-scale digesters that are fed on a semi-continuous basis.That work demonstrated two things. First, at 35°C, A. suumova inactivation was found to be weakly dependent onfeeding frequency (i.e., inactivation only increased slightly,from 61 to 64% when the feeding interval was increasedfrom 2 to 7 days). In contrast, the model showed that A.suum ova inactivation was more dependent on solidsretention time (i.e., inactivation increased from 40 to 73%when the solids retention time was increased from 15 to 45days). The second item demonstrated in this study wasrelated to field operation that would result in safe use oftreated sludge that contained persistent pathogens such asAscaris. In this case, regardless of the feeding interval, it isexpected that some percent of the digester effluent will

remain in the reactor less than the time required toinactivate the ova. Thus, one can’t ensure the digestereffluent is completely safe in these reactors, unless anupper mesophilic temperature is maintained or the feedinginterval exceeds the critical time needed for completeinactivation.

Literature thus suggests that to ensure inactivation ofpersistent pathogens like Ascaris spp. in an intermittentlyfed digester operated under mesophilic conditions, a usermay want to add a second digester of equal dimensions andalternate use of the two digesters. The first digestercontents would then be left in a storage mode after itreaches capacity, then the user would revert to the seconddigester. This strategy is typically employed for operationof dual compost latrines (Mehl et al., 2011). It has also beensuggested that in single state digesters with a fixed solidsretention time, differences in reductions of bacteria andviruses may occur if the reactor is operated using a “drawdown, then fill” feeding protocol versus a “fill, then drawdown” feeding protocol (Farrell et al., 1988). In that study,the impact of feeding protocol was observed to be greaterfor bacterial indicators than viruses (i.e., 1-1.5 log10

difference for bacterial indicators versus 0.24 log10

difference for viruses). Effluent from these reactors canalso be further treated in sludge-drying beds or humanexposure can be reduced by burying sludge to reducehealth risks by minimizing exposure (as discussed in theConclusions).

4.2 Chemical Treatment

Chemical treatment can consist of adding an alkalinematerial or ammonia to sludge. The most typical chemicaltreatment method is to stabilize sludge by adding analkaline material such as quicklime (i.e., calcium oxide orCaO), hydrated lime (i.e., calcium hydroxide or Ca(OH)2),lime containing kiln dust, or fly ash with the sludge. The kgof hydrated lime required per 1,000 kg of dry sludge toraise the pH above 12 is reported by Malina (1993) andaverages 110 kg for primary sludge, 270 kg for activatedsludge, and 170 kg for anaerobic digested sludge.

The best removal of pathogens with addition of alkalinematerial is obtained if the pH is raised to at least 12 for 72hours (Andreoli et al., 2007). Long term storage of thesludge treated in this manner requires the pH to remainabove 12 to ensure there is no regrowth of pathogenicorganisms. In full scale study in France, no viableenterovirus or genome was detected in sludge treated aftermechanically mixing with quicklime at a proportion of 50%dry matter to the biological sludge, which has previouslyundergone thickening and dehydration (Monpoeho et al.,2004).

Aqueous phase f ree ammon ia i s l e tha l t omicroorganisms. It is found in digesters, especially athigher pH and temperatures. Pathogens can also bereduced by adding ammonia directly to the sludge, by useof urea or urine. Aqueous phase free ammonia greater than80 mg NH3/L has been reported to be lethal to Ascarissuum ova (Cruz-Espinoza et al., 2012; Pecson et al., 2007;Nordin et al., 2009). This technology has been proposed for

http://www.waterpathogens.org/sites/default/files/figure%203_0.jpg


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management of fecal sludge (e.g. Trimmer et al., 2016). 4.3 Composting

The composting process is discussed by Andreoli et al. (2007) and Strande et al. (2014). Andreoli (2007) and Mihelcic et al. (2009,2014) provide examples on design and calculation of the proper mixture to achieve a sufficient carbon-to-nitrogen ratio. Compostingcan be performed using one of three methods: 1) aerated static pile, 2) in-vessel, and 3) windrow. Windrow is the less complex andmore traditional process. Composting has relatively low capital costs, but it requires a larger amount of land area and may generateodorous air if not properly aerated. A video of composting sludge with plant waste using mechanical turning is shown here:https://www.youtube.com/watch?v=3RzPIuSNykE. Compost can also be turned manually.

Figure 4. Windrow composting allows for mixing of the components (photo from U.S. Department of Agriculture,Agricultural Research Service Information Staff). The figure shows the composting of plant waste, not sludge; however, theprinciple is the same).

During windrow composting, a mixture of sludge andbulking agents is placed in rows (called windrows) 1-1.8 mhigh and 2-2.5 m wide. The windrows are mechanicallyturned at regular intervals for a minimum of 15 days(USEPA, 1994). Samples are then stabilized further so thetotal time of composting and final stabilization may be50-90 days.

During the composting process, dewatered sludge(Andreoli et al. (2007) suggests a solids concentration of >35%) is mixed with bulking agents (e.g., municipal bulkingagents of market , house, and park waste) andmicroorganisms degrade organic carbon in the compostpile and in the process generating heat. This thermophilicprocess can attain temperature of up to 80°C (idealtemperatures are 55-60°C, higher temperature andincreased contact time result in better destruction ofpathogens). Temperatures of 40-60°C can be achievedwithin the first three days (Andreoli et al., 2007). As thereadily available organic carbon is degraded, temperatureswill fall to 55°C or less. One recommendation to reducepathogens during windrow composting of sludge is tomaintain a temperature of ≥ 55°C for at least 15 days whileturning the windrow at least five times (USEPA, 1994).Maintaining an appropriate pH is usually not a criticalfactor if the carbon-to-nitrogen ratio is adequately set

(Fernandes, 2000). One issue not well studied is whetherthese higher temperatures are achieved in small scalecomposting operations that don’t employ mechanicalmixing.

The key to increasing destruction of pathogens duringcomposting is to ensure the thermophilic compostingprocess achieves higher temperature by: 1) maintaining asuff ic ient supply of oxygen (through stat ic ormechanical/manual aeration), 2) maintaining moisturecontent between 50-60%, and 3) raising and maintainingthe carbon-to-nitrogen ratio to 20-30:1. Important tosuccessful operation is the fact that temperature andmoisture content are relatively easy parameters to monitor.In addition, because the heat produced during thecomposting process is lost to the surrounding environment,higher compost temperatures may be more difficult tomaintain in cooler climates. Andreoli et al. (2007) reportthat batch composting systems require 12-30 m3 of air perhour per kg of dry mixture at the beginning of the process.This rate of aeration may reach as high as 190 m3 of air perhour per kg of dry mixture as the thermophilic organismsgrow. In natural aerated systems, it is recommended to mixthe contents at least five times while the organics are intheir thermophilic growth phase.

https://www.youtube.com/watch?v=3RzPIuSNykE


14

Because municipal wastewater sludge and feces have acarbon-to-nitrogen ratio of 5-10:1, carbon rich materials(i.e., bulking agents) are added to create an appropriatemixture. Examples are provided in Andreoli et al. (2007)and Mihelcic et al. (2009) on how to achieve appropriatemixtures of sludge and other additives. These additives alsoprovide better porosity to facilitate aeration of the compostpile. Carbon rich bulking agents can be municipal wastessuch as market, yard, and park waste but may also includeagriculture and industrial byproducts such as wood chips,wood bark, sawdust, straw, and rice hulls. Carbon-nitrogenratios for different bulking agents are available inreferences mentioned earlier and on-line. Fecal sludge fromcomposting latrines need to be carefully monitored forpresence of pathogens as many latrines, as operatedcurrently, may not achieve temperature much greater thanambient (see Mehl et al., 2011). Thus post treatment offecal sludge from composting latrine by increasingtemperature has been shown to further reduce virus andAscaris concentrations (Darimani et al., 2015).

Bacterial and viral pathogens can be reduced 1-2 log10

units if composted at 40°C or higher for up to 5 days.Reduction of bacterial indicators as high as 2-5 log10 may beachieved in composting (Sidhu and Toze, 2009). It isbeneficial that for 4 hours during this 5-day period thetemperature of the pile exceeds 55°C (USEPA, 2003). Inone study, no enteroviruses were cultured from sludge thatwas composted for 3-4 weeks (at temperatures that rangedfrom 30-68°C) and then left for a further 3 months in amaturation rack (Monpoeho et al., 2004). Insufficientresearch on the fate of helminths and protozoa in sludgecomposting systems was identified for this chapter.

Fecal sludge has been treated using vermicomposting.Vermicomposting consists of using worms that grow on thefecal sludge and produce a residue referred to asvermicompost that is reported to be high in nitrogen andphosphorus. Moisture levels much be maintained at 65-85%by addition of dry bulking agents and treatment time can

exceed three months (Semiyaga et a l . , 2015) .Vermicomposting has been reported to reduce pathogenlevels found in wastewater biosolids. For example, a fieldexperiment using vermicomposting to treat wastewaterbiosolids that was applied in windrows (6 m long by 1.5 mwide and 20 cm deep). Worm biomass was then added at aratio of 1:1.5 wet weight earthworm biomass to biosolids.In that particular study the following reductions wereobserved: 6.4 log10 reduction in fecal coliforms, 8.6 log10

reduction of Salmonella spp., 4.6 log10 reduction in entericviruses, and 1.9 log10 reduction in helminth ova (Eastman etal., 2001).

4.4 Thermal Treatment

Thermal treatment (i.e., heat drying) can be performedon primary or stabilized sludge. Treatment of fecal sludgeby mechanical dewatering and thermal drying can befollowed by pelletizing to produce a final product that canbe used as a soil conditioner and fertilizer (Strande et al.,2014). It is feasible when performed on dewatered sludgeand if the influent solids content is 15-30% solids. Reducingwater content prior to heat drying is very importantbecause 2,744 kJ of energy are required to evaporate 1 kgof water contained in sludge (Andreoli et al., 2007).Thermal treatment can also result in reduction of sludgevolume, reduced storage and transport costs. Also, nospecial equipment is required for land application of theprocessed biosolids. However, thermal treatment is anincineration process that requires input of external energy.High temperatures can be reached in thermal treatmentprocesses such as fluidized bed dryers and drum dryers(Table 7) and will ensure the sludge is virtually free ofpathogens and meets health standards for less restrictedland applications. However, the temperatures reached inother systems, such as belt dryers, may not necessarily behigh enough to destroy all types of pathogens (Figure 5),given the short amount of time sludge is retained in thesesystems (typically < 1 hour).

Table 7. Typical temperatures achieved in common thermal treatment technologies for sludge (data fromBurton et al., 2014)

ThermalTreatment

(HeatDrying)

Technology

TypicalOperation

Temperatures(ºC)

Belt Dryer < 90FluidizedBed Dryer 120

Other DirectDryers 370


15

Figure 5. Reduction of bacterial and viral pathogen surrogates in sludge during thermal treatment at 80°C (data obtainedfrom Mocé-Llivina et al. 2003).

4.5 Thickening and Dewatering

Thickening of sludge has been applied to sludgeassociated with primary treatment, activated sludge,trickling filter processes, and fecal sludge. Dewateringprocesses and sludge drying are applied to digested sludge(Andreoli et al., 2007). Sludge drying can be applied to anytype of sludge.

3.5.1 Thickening

The purpose of sludge thickening is to reduce the watercontent of the sludge and the inherent time expenseassociated with transporting or thermally treating thewater found in sludge. Another reason to reduce the watercontent of sludge is to decrease the volume of sludgerequired for land application or disposal. Sludge thickeningis typically accomplished by: 1) gravity thickening and 2)dissolved air flotation thickeners. Sludge thickening doesnot reduce water content enough that it would be expectedto inactivate pathogens to any significant manner.

A gravity thickener looks similar to a circular gravitysedimentation tank. The influent sludge is applied to thecenter and thickened sludge is removed from a withdrawalmechanism located in the bottom of the tank. Design of asludge thickener is based on solids loading rate (kg totalsolids (TS)/m2-d) and hydraulic loading (m3/m2-d) (Andreoliet al., 2007). Dissolved air flotation technologies aretypically applied to sludge derived from the activatedsludge process or in treatment plants that incorporatebiological phosphorus removal. In this process, air is forcedinto the l iquid sludge under high pressure. Asdepressuration occurs, the dissolved air forms gas bubbles

that rise to the surface, carry particulates to the surfacewhere they can be skimmed off.

No relevant data on pathogen inactivation duringsludge thickening processes was identified, and it isassumed that this process makes a negligible change in thelevels of pathogens in sludge.

4.5.2 Mechanical dewatering

Mechanical dewatering methods include technologiessuch as a: 1) belt filter press and 2) centrifuge. Theirpurpose is to reduce the water content of sludge but notenough that they would be expected to inactivatepathogens. A belt filter press is a mechanical device wheresludge is added to a moving belt at ambient temperaturesand that is squeezed to remove water. A centrifuge consistsof a bowl where solids are moved to the wall by centrifugalforce and scraped out by a screw conveyor (Mihelcic andZimmerman, 2014; Strande et al., 2014).

A study from five treatment plants in the U.K.(Monteleone et al., 2004) reported that for mesophilicanaerobically digested sludge that was dewatered bymechanical methods, most of the centrifuge dewateringsystems concentrated E. coli in the sludge and E. coliconcentrations did not increase when the sludge wasdewatered by a belt filter press. Variations in E. coliconcentrations across different mechanical water systemsthat may be dependent on the origin of the sludge and thespecific dewatering process. Centrifugation appeared toincrease E. coli concentrations by a little over 1 log10 unitwhile a belt filter process was found to have a similarreduction in E. coli (Monteleone et al., 2004). It was


16

believed the increases in centrifuge indicator countssuggests that physical forces during dewatering causedisruption of floc structures which releases microorganismsassociated with solid particles.

A quantitative microbial risk assessment (QMRA) thataccounted for rotavirus, adenovirus, haemorrhagic E. coli,Salmonella, Giardia and Cryptosporidium was performed ata Swedish 12,500 m3/d treatment plant that containedtertiary wastewater treatment and mesophilic sludgedigestion. The QMRA showed the greatest individual healthrisk from a single exposure was to workers exposed toaerosols produced at a belt press used for sludgedewatering (virus infection risk = 1). The largestcommunity impact was determined to arise if childreningested sludge at an unprotected storage site (Westrell etal., 2004).

4.6 Dewatering by Drying

4.6.1 Sludge drying beds

Dewatering of sludge can also occur through waterevaporation, plant transpiration, or vertical transport ofwater out of a layer of sludge spread over a drainage layer.There are many technologies that employ dewatering bythese three mechanisms. Some examples are 1) convectiondrying, 2) contact drying, 3) open sludge drying beds, 4)reed beds, and 5) solar drying. Because they utilize similarfundamental properties, not all technologies are describedhere. Dewatering technologies are expected to reducebacteria levels by 2 log10 units and pathogenic virusconcentrations by 1 log10 unit (USEPA, 2003). To assistoperation of dewatering technologies discussed in thischapter, Table 8 provides information on how to interpretvisually inspected dewatered sludge in regards to themoisture content. Unfortunately, this visual inspection ofdried sludge does not provide sufficient insight on pathogendestruction.

Table 8. The meaning of visually inspected sludge summarized (from Imhoff and Hess (1966) using Andreoli etal., 2007). Sludge changes from a liquid to a solid at 18-20% solids.

VisualInspection

What itmeans

Dry sludgewith scarceand thincracking

Well-digestedsludge withlow watercontent

Largenumber ofmedium-sizedcracks

Digestedsludge withhigh water

content

Smallquantities ofwide cracks

Poorlydigested

stickysludge,

requires along drying

period

Sludge drying removes water by evaporation, transpirationby use of deep rooted plants, and transport of watervertically from a drying bed to an underlying drainagelayer. Sludge dried in this manner may have been pre-treated (i.e., stabilized) prior in an aerobic or anaerobicreactor. Sludge is placed in layers not exceeding 20-30 cmover a drainage layer and supporting foundational layeruntil the solids content is reduced to 30%. A mechanical ormanual system can be used to turn the sludge. The dryingbed may be lined with sand layers on top of concrete, andbe open or closed to protect against rainfall. Greenhousescan be used as a cover to provide a barrier to rainfall andalso increase temperature. Sludge is more easily removedfrom a drying bed when the solids content reaches 30%.Pathogens are removed in a drying bed by a combination ofdesiccation and UV disinfection. Drying obviously occurs

better in dry and warm climates and much less in wet andcold climates.

Planted beds can improve dewatering throughtranspiration provided by deep rooted plants that also helpwith vertical drainage by roots. Reed bed technology usereeds (tall grasses that prefer marshy locations). The use ofplants in dewatering sludge is described later in thischapter in the section that discusses desludging awastewater lagoon.

4.6.2 Solar drying

Solar drying systems consist of a drying bed that inhigher rainfall areas can be enclosed in drying chamber(similar to a greenhouse). Sludge can be turned andaerated by manual labor (using proper protective


17

equipment) or mechanical methods. The enclosed systemsmay have a mechanical ventilation system installed.Moisture is evaporated from the system to achieve a finalsolids concentration of 70-90%.

Short wavelength UV light has been found to beeffective in reducing pathogen concentrations (Sinton et al.,2002); however, short wavelength UV light is blocked bythe sludge layer and many commercial greenhouse roofsfound in enclosed solar dryers. Therefore, pathogen die-offmechanisms in an enclosed solar dryer are most likely aresult of a reduction in moisture content and exposure tohigher temperatures. In one study of a full scale system(Australia), temperatures and moisture content weremeasured within the sludge along different locations in thesolar dryer. Those results showed that the moisture contentwas reduced from 80% to 30-58% and a temperature of67.5°C was recorded within the sludge. The authors alsoreported that though fecal coliforms were identified in thetreated sludge, enteroviruses, parasites, E. coli, andSalmonella sp. were not detected in the solar dried product(Shanahan et al., 2010).

4.6.3 Desludging a waste stabilization pond

Removal of accumulating sludge from a wastestabilization pond is important to maintain proper hydraulicconditions which impacts pathogen removal (Ouedraogo etal., 2016). A general rule of thumb is to remove sludge fromanaerobic ponds every 2-5 years and from a facultativepond every 5-15 years. Unfortunately, waste stabilizationponds are often designed and operated withoutconsideration of desludging. Regarding design, forlocations that use dry desludging methods (discussedbelow), ponds be designed with a slope towards thedischarge side to promote water draining away from sludgethat accumulates on the intake side. Furthermore, pondsare typically operated without the cost of desludgingincluded in the operation and maintenance budget (Oakleyet al., 2012).

Ponds can be desludged by wet or dry removal methods.Wet removal requires the use of sludge pumps, vacuumtrucks, and dewatering equipment (Carré et al., 1990; Picotet al., 2005). Dry removal methods are less expensive andone method has been demonstrated in detail by Oakley etal. (2012). Greater details on sludge removal from wastestabilization ponds is available in Carré et al. (1990),Goncalves (1999), Picot et al. (2005), Von Sperling (2007),and Oakley et al. (2012).

Dry removal methods require that the sludge first be driedto a dry solid (18-20% solids). Sludge changes from a liquidto a solid around this percent solids and after it becomes asolid it won’t decrease in volume much more. The reason todry the sludge first is is because heavy equipment willbecome stuck in deep sludge that is not dried to a solid. Ifthe influent is routed away from a pond and the contentsare allowed to drain so the sludge dries, deeper sludge willnot dry because a crust will form on the surface that mayextend to depth of 0.15 to 2 m. This crust will prevent thedeeper sludge from drying.

Figure 6. Waste stabilization pond sludge removalfrom waste stabilization pond in Tela, Honduras. Inthe upper photo an excavator removes surface anddeep sludge (up to 1 m) from pond planted with deeproot wetland plants. In total, 3,000 m3 of dried sludgewas removed in 5 days with the excavator (after pondwas dried by draining and planting of deep rootedwetland plants) (photos provided by Stewart Oakleywith permission).

Dewatered sludge will contain pathogens, includinghelminths. Care thus needs to be made to ensure humansand livestock do not come in contact with the dewateredsludge. Pond designs should thus include reserving an areawhere excavated sludge can be safely stored for ≥ 1 year tomeet the WHO guidelines for inactivation of helminth eggs(WHO, 2006).

4.7 Other Treatment Methods

Fecal sludge has been treated using black solider flies(Hermetia illucens). A comprehensive manual on use ofblack soldier flies to process organic waste is available(Dortmans et al., 2017).

Black soldier fly larvae have been shown to grow onorganic wastes such as fecal sludge. The larvae aerate themixture and the resulting compost can be used as a soilconditioner. Some have suggested that the fly prepupaethat are produced during composting could also be used toproduce animal or aquaculture feed (Gabler, 2014) becausethe prepupae consist of 40% protein and 30% fat (Sheppardet al., 2002). This technology has been reported inlaboratory experiments to reduce Salmonella spp. by 6 log10

in eight days. No reduction was observed however over thatsame time period for Enterococcus spp., bacteriophageΦX174 or Ascaris suum ova (Lalander et al., 2013).

5.0 Additional Information on Fecal SludgeManagement

As mentioned in the chapter’s Introduction, fecal sludgeis a distinct type of wastewater sludge that is generatedonsite (e.g., discharge to toilet/latrine pits and septic tanks)and is not transported through a sanitary sewer. Manydocuments report specifically on fecal sludge management(e.g., Strande et al., 2014; Semiyaga et al., 2015; WorldBank, 2017). Fecal sludge has different physical-chemical

http://www.waterpathogens.org/sites/default/files/figure%206_0.jpg


18

characteristics than sludge generated at a centralizedtreatment plant. These particular characteristics have beensummarized elsewhere (Strande et al., 2014). Fecal sludgethat has aged in a pit or tank is believed to have reducedpathogen levels in because of aerobic and anaerobic

biological processes that occur in the pit. Furthermore,helminth egg concentrations are reported to be muchhigher in sludge collected from toilet/latrine pits and septictank waste and raw sewage sludge (Semiyaga et al., 2015).

The service chain for fecal sludge management is provided in Figure 7. As shown in Figure 7, there is a potential for increasedhuman health risk associated with exposure to pathogens present in the fecal sludge during the collection of the material andsubsequent transport to a treatment or disposal facility. Thus design and management of toilet/latrine and septic systems in rural andurban areas must also consider safe collection and transport of the contents. This differs from a system where excreta is collected atthe point of production and transported in a secure sanitary sewer to a centralized wastewater treatment plant. Sanitary sewershowever are not perfect because they are known to develop cracks and can leak their contents to groundwater and surface water.They also come with added economic and environmental costs. For example, an economic comparison of a sanitary sewer connectedto an activated sludge treatment system (a relatively more expensive sanitation technology) was compared to septic tank systems thatwere integrated with collection and transport of fecal sludge that was then treated in drying beds. That study (Dodane et al., 2012)showed the combined capital and operating costs for the centralized activated sludge system were five times greater than thedecentralized onsite system with fecal sludge management. Bathroom and collection infrastructure have also been shown to havehigher energy intensity (and thus carbon footprint) than associated treatment technologies such as waste stabilization ponds andupflow anaerobic sludge blanket reactors (Cornejo et al., 2013).

Figure 7. Fecal sludge sanitation service chain (reprinted from Water, Sanitation & Hygiene Strategy Overview, Bill &Melinda Gates Foundation, 2012)

In addition, fecal sludge is widely produced around the world, but is currently poorly managed. For example, in a study of twelvecities, approximately 66% of urban households were found to rely on onsite sanitation technology (WSP, 2014). These onsite systemswould thus generate fecal sludge. However, only 22% of the fecal sludge produced in these 12 cities was estimated to be safelymanaged. One reason for this is because as currently practiced, fecal sludge management is mostly unplanned and managed bynonregulated informal private entities (WSP, 2014). To demonstrate the importance of the health implications of improper fecal sludgemanagement, Figure 8 shows that for the example of Dakar, most human excreta is contained at the house level. However, as the fecalmatter moves through the sanitation service chain, there is a large loss of it (and pathogens associated with the waste) to the urban

the Water and Sanitation Program, The World Bank Group, 2014).Figure 8. Fecal Waste Flow Diagram for Dakar (Senegal) (reproduced with permission from a Research Brief developed by


19

environment. This example is not to say that decision makers should advocate only for centralized systems. This is because onsitewastewater management will have a continued role to play in rural and urban settings and sludge produced at a centralized treatmentplant can also be unsafely managed.

After it is collected and transported, the fundamental processes of pathogen fate and the treatment technologies applied to fecalsludge are similar to processes and technologies described in Sections 3 and 4 of this chapter. Figure 9 shows how collection andtreatment of fecal sludge is currently practiced in some parts of the world. Strande et al. (2014) provide design of many examples fortechnologies to treat fecal sludge.

Figure 9. a) Transport of fecal sludge from an informal settlement in Nairobi. b) Addition of water to pit latrine to aidremoval of pit contents in Nairobi, c) drying bed for treatment of fecal sludge in Dakar, d) biogas reactors used tostabilize fecal sludge in Ouagadougou, (photos (a-d) provided by Linda Strande with permission) pilot composting facilitythat combines fecal sludge and municipal solid waste in Bangalore (photo (e) provided by Christian Zurbrügg withpermission).

6.0 Data on Pathogen Reduction in SludgeManagement Systems

The removal of pathogens during sludge managementclearly depends on a number of environmental factors aswell as engineering design and operation and maintenancepractices, as demonstrated in the previous sections. Datafrom a review of the literature have been collected about

pathogen removal during sludge management. Thefollowing four factors were found to have the largest impacton pathogen removal: 1) retention time, 2) temperature, 3)pH, and 4) moisture content. Sunlight exposure is notexpected to result in significant removal of pathogens.These four factors are summarized in Table 9. Table 10provides a summary of typical pathogen removalefficiencies for specific technologies used for sludgemanagement.


20

SludgeTemperature Higher Temperature = ↑ Pathogen Removal Xa X X X

RetentionTime Longer Retention Time = ↑ Pathogen Removal XX XX XX XX

pH Higher pH = ↑ Pathogen Removal XX XX X XFreeammoniaconcentration

Higher NH3 concentration = ↑ Pathogen Removal XX XX XX XX

MoistureContent Lower Moisture = ↑ Pathogen Removal XX XX XX XX

Sunlight (UV)Exposure

Sunlight (UV) exposure is not expected to be a significant removalmechanism for most technologies used for sludge management

unless it raises temperature or results in desiccation of the sludge

a X means less impact XX means more impact

Table 10. Summary of indicator and pathogen removal for specific sludge management technologies

SludgeManagementTechnology

ExpectedIndicator

andPathogen

Destruction

Organisms Log10

Reductions Reference

AerobicDigestion

Thermophilicdigestion is

more efficientat removingpathogens

thanmesophilicdigestion

Helminth ovaare reducedto varyingdegrees,

depending onthe hardiness

of theindividual

species

Fecal indicators,bacterial

organisms andviral pathogens

1 USEPA,2003

HelminthsProtistsBacteriaViruses

Pathogen GroupFollowing ConditionsPathogen Removal is ↑ Enhanced or ↓ Reduced Under theFactor

Table 9. Summary of key factors and strategies to enhance pathogen removal in sludge management


21


ExpectedIndicator

andPathogen

Destruction

Organisms Log10


Anaerobicdigestion

Thermophilicdigestion is

more efficientat removingpathogens

thanmesophilicdigestion.

Bacterialorganisms

Viral PathogensFecal indicators

21

1 to 2

USEPA,2003

No observedimpact

observed onGiardia cystswas found in

one study

Giardia lambliaCryptosporidium

parvum0.551.56

Kinyua etal., 2016

In a study of18 large full-

scalemesophilicanaerobic

digesters inthe U.S. the

averagereductions

Enteric virusesSalmonella

Fecal coliformsTotal coliforms

1.251.621.961.92

Pepper etal., 2010

If thetemperatureis raised to 50°C virus

inactivation isreported to

reach 99.99%

Viruses 4 Sanders etal., 1979

Composting

If compostedat 40°C or

higher for upto 5 days

Insufficientdata availablefor helminthsand protozoa

Fecal indicatorsBacteriaViruses

2 to 51 to 21 to 2

USEPA,2003

Heat Drying

Temperaturesabove 80°C

can beachieved in

most of thesesystems,

which wouldensure that

sludge isvirtually freeof pathogens

and meetshealth

standards forless

restrictedland

applications.


22


ExpectedIndicator

andPathogen

Destruction

Organisms Log10


ChemicalStabilizationby Addition ofAlkalineMaterials

Fecalcoliforms arereduced tobelow thelimits of

detectiononly 4 hoursafter adding

quicklime(CaO) to

sludge at aconcentration

of 2%

After 14 dayswith a

quicklimedose of 8%.

Ascaris eggs

in sludge mayretain

viability, butwere unableto developpast the

unicellularstage aftertreatment

withquicklime at

10%;

Exposure toquicklime for

5 monthsresulted intheir loss of

viability

C. perfringensvegetative cells 3 Andreadakis,

2000

The additionof lime at a

ratio of 0.3 glime for each1 g of waste

activatedsludge 5 days

reducedviable ova

from 89 to 22ova/dry g (0.6

log10 units)

Fecal coliformsHelminth eggs

>40.5

Farzadkiaand

Bazrafshan,2014


23


ExpectedIndicator

andPathogen

Destruction

Organisms Log10


ThermalTreatment

Hightemperatures

can bereached in

thermaltreatmentprocessessuch as

fluidized beddryers and

drum dryersthat will

ensure thesludge is

virtually freeof pathogens

and meetshealth

standards forless

restrictedland

applications.

A solarconcentrator

has beendemonstrated

to increasetemperature

of faecalsludge;

reductions inone day

Helminth eggs 2.96 Foote et al.,2017

Dewatering(Drying Bedsand SolarDrying)

No literaturewas identified

for otherpathogens.

Fecal indicatorsBacteriaViruses

1 to 221

USEPA,2003

ThickeningandMechanicalPresses

Sludgethickening isnot expectedto reduce theconcentrationof pathogens

to anysignificantmanner.

7.0 Conclusions

During sludge processing, pathogens are inactivated bydifferent mechanisms. The efficiency of these mechanismsis dependent on a number of environmental, design, andoperational factors. Temperature and time have beenidentified as perhaps the two most important factors inachieving significant pathogen destruction during sludgemanagement (Sahlström, 2003) and pH and moisturecontent are also important. Examples of sludge

management technologies that can achieve temperaturesgreater than 45°C if properly designed and operated are: 1)composting, 2) thermal treatment, and 3) thermophilicdigestion. For lower temperature treatments, pathogenconcentrations are reduced but still routinely identified intreated sludge (Gantzer et al., 2001).

Other ways to improve pathogen destruction duringsludge treatment are reducing moisture content by drying(through evaporation, transpiration by use of deep rooted


24

plants, and drainage of water vertically from a drying bed),raising pH, and/or increasing the concentration of free(unionized) ammonia. Sunlight’s role in decreasingpathogen concentrations in sludge is mostly throughsupporting desiccation of drying sludge (USEPA, 2003).Biological processes (e.g., digestion, composting) canreduce the volatile organic fraction of sludge; thus,reducing disease vector attraction and bacterial growthbecause of loss of available organic substrate (USEPA,2003).

Table 11 provides recommendations made by the WorldHealth Organization (2006) for storage treatment of driedexcreta and fecal sludge. Note the information provided inthis table shows the importance of storage time,temperature, and pH in reducing pathogen levels. TheWorld Health Organization also recommends an additionalbarrier of safety of waiting one month between the time thestored excreta is applied to a crop and the time the crop isharvested.

Table 11. World Health Organization (WHO) recommendations for storage treatment of dry excreta and fecalsludge (reprinted with permission from Table 2 (Page xvi) from the WHO 2006 Guidelines for the Safe Use ofWastewater, Excreta, and Greywater Volume 4 Excreta and Greywater Use in Agriculture, 2006.)

Treatment Criteria CommentStorage atambienttemperatureof 2 to 20°C

6 monthsto 2 years

Eliminates bacterial pathogens, regrowth of E. coli and Salmonella may need to be considered if rewetted; willreduce viruses and parasitic protozoa below risk levels. Some soil-borne ova may persist in low numbers.

Storage attemperatures> 20 to 35°C

> 1 yearSubstantial to total inactivation of viruses, bacteria, and protozoa; inactivation of schistosome eggs (< 1 month);

inactivation of nematode (roundworm) eggs; survival of a certain percentage (10 to 30%) of Ascaris eggs (≥ 4months), where a more or less complete inactivation of Ascaris eggs will occur within 1 year.

AlkalineTreatment

pH > 9for > 6months

If temperatures > 35°C and moisture is < 25°C, lower pH and/or wetter material will prolong the time for absoluteelimination

Helminth eggs are particularly resistant to sludge treatment processes, including thermal and chemical treatment. Therefore, inregions where helminths are endemic, it is recommended that sludge be reused with extreme caution, especially given that fewlaboratories in these regions are equipped to monitor helminths in sludge on a routine basis. In regions where helminth infectionsregularly occur, it is recommended to manage sludge in these settings to provide simple treatment in sludge-drying beds, and then tobury the dried sludge to further reduce health risks by minimizing exposure (Figure 10).

Figure 10. Example of safe management of sludge in a small town in southern Brazil with sludge drying beds (upper left),emptied manually by personnel using protective equipment (lower left); dried sludge is then buried onsite in pits (upperand lower right) (photos provided by Matthew E. Verbyla with permission)


25

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Andreadakis, A.D. (1999). Treatment and disinfection of sludge using quicklime. DGE/JRCEC European CommissionWorkshop Around Sludge, Italy, Stresa. pp. 18–19.

Andreoli, C.V., von Sperling, M., Fernandes, F. and Ronteltap, M. (2007). Sludge treatment and disposal. IWA Publishing.London, United Kingdom.

Arthurson, V. (2008). Proper sanitization of sewage sludge: a critical issue for a sustainable society. Applied andEnvironmental Microbiology. 74, pp. 5267–5275.

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Burton, F.L., Stensel, H.D. and Tchobanoglous, G. (2014). Wastewater engineering: treatment and resource recovery. 5thed. McGraw-Hill.

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