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Physico-chemical Properties of Biosolids Produced by Aerobic Liquid Digestion Employing the

ThermAer Second Generation ATAD Process

Research Report

Submitted to

WCI Environmental Solution Inc. by

Prof. Yona Chen and Dr. Jorge Tarchitzky

Department of Soil and Water Sciences

Faculty of Agriculture, Food and Environment

The Hebrew University Jerusalem

POB 12, Rehovot 76100, Israel

Rehovot, Israel January 2009

Physico-chemical properties of Biosolids produced by aerobic liquid digestion employing the "ThermAer" process

Expert's opinion and Summary Autothermal Thermophilic Aerobic Digestion (ATAD) is a process involving the

controlled and intense aeration of organic slurries in an enclosed, insulated bioreactor

under fully aerobic conditions (analogous to liquid composting). Thermal Process

Systems of Indiana have developed a second generation ATAD system (ThermAer),

with twelve plants currently operating in the US and two facilities under development

in Canada. ThermAer ATAD products (dewatered sludge cake) from three different

treatment plants were the subject of investigation in this research project. The

ThermAer ATAD process generates an EPA Class A, malodour free product.

Ready to market commercial composts can vary greatly in their composition,

quality and degree of maturity. Four composts were studied in order to compare three

biosolid composts processed in a 2nd generation "ThermAer" systems to a biosolid

compost from an open windrows system. The results of the analyses were discussed in

relation to maturity indices and compost composition in order to evaluate the

materials in relation to their agronomic quality.

Percentage of the organic matter (OM) varied between 30% to 56%. The

"Bowling Green" compost exhibited a remarkably low OM content and consequently

high ash content. The low OM and high ash content of this compost is the result of the

thickening process which includes the addition of bentonite to the compost. All the

composts are in a range of OM accepted and authorized for biosolid composts.

The dissolved organic carbon (DOC) concentration in the water extract of the

composts decreases during the composting process and has been defined as a good

index of compost maturity. A concentration of 4 g kg-1 has been proposed to be a

threshold value for highly mature composts. .The DOC concentration for the

"Bowling Green" compost is lower than the threshold value and indicates compost at

the end of the maturation stage. In addition to differences in the process this is

probably the result of clay addition (see above). The other three samples have values

in the range of 6.7-7.7 g kg-1. These values are higher than the threshold value and

may indicate composts at a lower maturation degree. It should be noted however, that

field application of compost does not require maturity, since stability is sufficient for

storage, transport and field application.

1


The pH values for the various composts range from 6.78 to 7.23. A value of pH

in the range 6.5 to 7 is considered an indicator of a proper and full composting

process.

The EC of composts ranged from 0.78 to 3.35 dS m-1. These EC values are low

and suitable for agriculture use.

The N-NO3 concentration increases in the last steps of the composting process

as a result of aerobic conditions and achieves values higher than the very low

concentrations which are typical to raw sludge The concentrations measured in the

composts tested were in the range of 28.5 to 1127.9 mg L-1 N-NO3. The N-NH4

concentration decreases in the last steps of the composting process as a result of

aerobic conditions and a nitrification process The concentrations measured in the

composts tested were in the range of 81.4 to 160.4 mg L-1 N-NH4. These values are

characteristics of an early maturation stages.

Bicarbonate concentrations in the water extract of the composts were in the

range of 41.3 to 235.7 mg L-1. The lower value observed in the "Bowling Green"

compost is in agreement with the lower DOC concentration measured in the water

extract of this compost, and the higher level of maturity of this particular compost.

Micro-elements concentrations in the different composts vary without any

consistency based on source. The concentrations of all the elements were lower than

the requirements of the Israeli, European Union and USA-EPA regulations. Only the

copper concentrations in the "ThermAer" composts were higher than the very strict

Israeli regulation for this element. Yet, it should be noted that this is not a function of

the process but rather the source.

Boron concentration limit in the Israeli Compost Standard is 200 mg kg-1 dry

matter. This element which has toxic effect on plants, and is a very important

parameter when evaluating the agronomic suitability of the composts. All the

concentrations found in the tested materials in the present work are lower than the

threshold value.

Nitrogen contents in the composts ranged from 5.93 to 8.34%. All the materials

present higher N content in comparison to a series of composts from commercial

composting facilities in Europe. This is a positive property and is important to the

nutritional value of the compost in both agriculture and landscape utilization. The C/N

ratio in the materials tested (6.2-8.4) is characteristic of stable, or close to mature

biosolids compost. The H/C atomic ratio in the composts indicates aliphatic

2


compounds in the compost. The low SUVA values observed for the aqueous extract

of the composts can verify this assumption. The high O/H ratio in the biosolid

composts indicates that the organic matter in the compost is in a high oxidation stage.

The three samples from "ThermAer" contain negligible concentrations of

CaCO3 in contrast to the higher content in the Israeli compost. The difference can be

explained by the quality of the fresh water (high hardness) and the additions of yard

waste in windrow composting of biosolids.

All the composts tested in the current research exhibited similar DRIFT spectra

characteristics of biosolid composts. The ratio of aliphatic to aromatic or amide peak

was similar in all the samples, but lower than the ratio in manure composts, indicating

a decrease of the aliphatic peak relative to the aromatic or amide peak. These findings

are in accordance with the high presence of straw in manure composts in contrast to

the biosolid composts which are rich in protein.

In conclusion, the "ThermAer" composts are very similar to biosolid compost

processed at length to stability by an open windrows system. The composts are in an

early stage of maturation, yet they are very stable and odorless. In addition to the

chemical traits indicating stability they are stored for periods of months without the

development of offensive odors. As well they do not emit heat. These two facts

provide additional evidence for their stability which is an important asset to both the

producers and potential users. The high nitrogen content in the biosolid composts

represents an agronomic advantage along with low ash and CaCO3 content. All

composts meet the heavy metals concentration requirements. Thus the "ThermAer"

process which is short and efficient stands out as a very attractive option to

municipalities, while ensuring beneficial agricultural and landscaping utilization.

3


1. Introduction Compost stability and compost maturity are two terms often used to describe the

rate of the decomposition and transformation of the organic matter (OM) in compost.

Compost stability refers to compost in which the rate of energy release due to

microbial degradation of the OM equals the rate of energy loss to the environment.

Under these conditions, the temperature of the compost remains constant and equals

that of the ambient. Compost stability is strongly related to the rate of microbial

activity in the compost. Compost maturity is used to describe product quality. It is

based on a utilization oriented definition as follows: (i) greenhouse utilization: OM

composted to the degree of decomposition that has no adverse effects on container

grown plants; (ii) Field application: OM composted to the degree of decomposition

that has no adverse effects on growth of various crops when applied at annual rates up

to 50 tons ha-1 at least 6 weeks before sowing or planting (during the warm season in

which decomposition can take place) (Chen and Inbar, 1993).

As explained above, the term stability relates to the rate of decomposition of the

OM, as expressed by the biological activity of the compost. This is usually assessed

by different measures of the respiration rate, or by the self-heating of the compost in

standard conditions (Brinton et al., 1995, Adani et al., 2003). Usually maturity has

been evaluated based on chemical parameters. Since the use of the term relates to

compost quality, we believe that it should also reflect optimal utilization potentials,

namely, be correlated with plant response.

Using this definition, assessment of maturity could be straightforward, using

germination and plant growth tests. This is usually a tedious undertaking and there are

disagreements as to the real ability of germination tests to predict maturity (Warman,

1999, Brewer and Sullivan, 2003). Among the many papers published on compost

maturation, relatively few used plant growth tests to assess maturity (Inbar et al.,

1993, Iannotti et al., 1994, Grebus et al., 1994, Cooperband et al., 2003).

In some of the published research the relationship between plant growth and

compost maturity is not clear. This may be due to salinity problems or to traits of the

cultivars of the specific plant used.

The common approach in compost research is to examine the chemical or

biochemical parameters of the compost, and many such parameters have been

suggested as indexes of maturity. For a chemical parameter to serve as a predictor of

4


maturity it must change following a consistent trend during the composting process,

and reach a predictable threshold level at maturity. Some of the parameters often

referred to in composting literature are listed below and will be briefly discussed.

The widely used parameter for assessing compost stability is respiration, as

measured either by O2 uptake or CO2 evolution. Often respiration is also used as a

measure of maturity, especially by compost producers. Respiration must be measured

under well controlled conditions of moisture and temperature, and is affected by

temporary anaerobic conditions (Iannotti, 1994). In addition, Wu et al. (2000) showed

that low CO2 evolution is not always an indicator of non phytotoxic compost.

The C/N in the bulk compost decreases during composting regardless of the

composting technique employed. A ratio of 10 to 15 is considered stable, however it

may level off much before the compost stabilizes (Namkoong et al., 1999, Chefetz et

al., 1996) and the final ratio depends on source materials and on the method of N

measurement (Hue and Liu, 1995). The ratio of NH4-N to NO3-N in the water extract

has been suggested as an index of maturity. However, the final value of NO3- reached

depends on the source material. No particular level of NO3-, or its ratio to NH4 can be

relied upon as an indicator of compost biomaturity (Mathur et al., 1993; Bernal et al.,

1998a). It should also be noted that the increase in NO3- is gradual over a lengthy

period of time, thus the determination of the point at which the increase begins is

difficult. The water soluble organic C to organic N ratio (sol org C/org N) has been

suggested as a maturity index by several research groups (Garcia et al., 1991, Hue

and Liu, 1995, Bernal et al., 1998a, Fang et al., 1999). A threshold value of <5-6 for

mature composts has been reported by Chanyasak et al. (1983) and Jimenez and

Garcia (1989). Using this ratio as a maturity index is only possible when all organic

N species can be accounted for.

A variety of maturity indexes based on the humification process have been

proposed. They require measuring the alkali extractable fraction in the composts.

These include the amount of total humic substances (HS) as a percent of OM, the

humic acid (HA) fraction, the fulvic acid (FA) fraction, the ratio of HA/FA, and the

ratio of the non-humic fraction (NHF) to (HA+FA). Most of these indices show a

clear trend during composting, but their absolute values vary greatly among composts

of different source materials and their use therefore requires proper removal of the

non-humic fraction from the FA (Chen et al., 1996).

5


The dissolved organic carbon (DOC) or water soluble carbon has been proposed

by a number of researchers as a parameter which consistently decreases during the

composting process. The water to compost extraction ratio varies, as does the

definition of dissolved carbon. Several DOC cutoff values have been suggested as

indicators of maturity: Hue and Liu (1995) suggested 10 g kg-1 whereas Bernal et al.

(1998b) suggested 17 g kg-1. DOC was shown to be highly correlated to respiration

(Chica et al., 2003) and suggested as a maturity parameter after calibration of the

correlation per source material (Helfrich et al., 1998).

Zmora-Nahum et al. (2005) presented data collected on composts prepared in a

variety of processes with diverse source materials. Of the parameters tested on these

composts, the DOC stood-out in its consistent behavior and its correlation to plant

experiments, and suggested that the DOC concentration, which is a simple measure,

may be a sufficient parameter of compost maturity for composts of different source

materials and different composting processes, since it reaches a level of less than 4 g

kg-1 regardless of source materials and process. The absorbance at 465nm may be

used as a cheap and simple substitute for organic carbon determination, which can

also serve as a tool used by compost producers after calibration.

The aim of this research was to characterize the composts and evaluate the

stability of three compost products representing typical small/midsize second

generation Thermaer facilities active in Midwest states of the US. The properties and

stability of the composts are essential information when the feasibility and benefits of

their use in landscaping and agriculture are to be evaluated. For comparison we also

included in each of the tests conducted a typical windrow prepared biosolids compost

(1:1 sewage sludge:yard waste v/v) produced by the largest compost producer in

Israel (Shacham, Guivat Ada) which was further stabilized in Yona Chen`s laboratory

(Zmora-Nahum et al., 2008a).

The traits tested are listed in the Materials and Methods section. They were

selected based on there usefulness and relevance to the characterization of a

composting and stabilization process of composts in general, and biosolids in

particular.

6


2. Material and Methods 3.1 Source materials and the composting processes

Autothermal Thermophilic Aerobic Digestion (ATAD) is a process involving the

controlled and intense aeration of organic slurries in an enclosed, insulated bioreactor

under fully aerobic conditions (analogous to liquid composting). Thermal Process

Systems of Indiana have developed a second generation ATAD system (ThermAer),

with twelve plants currently operating in the US and two facilities under development

in Canada. ThermAer ATAD products (dewatered sludge cake) from three different

treatment plants were the subject of investigation in this research project. The

ThermAer ATAD process generates an EPA Class A, malodour free product.

The ISBS11 sample was originally sampled at the Shaham Guivat Ada-Dlila

composting facility, at a stage of "ready to market", namely after about 2.5 months of

windrow composting, the 97 days of stabilization process in a bin (Zmora et al.,

2008a).

Source materials are detailed in Table 1.

Table 2: source materials. Material Location Process Delphos belt cake @ 26.89.TS 10/2/08 Delphos, Indiana Yorkville 25.29.TS before drying 10/6/10 Yorkville, Illinois Bowling Green centrifuge 40% TC 10/2/08 Bowling Green, Ohio

ThermAer Second

Generation ATAD

ISBS 11 4/2/06 Israel Windrow compost

3.2 Chemical characterization

Composts from the USA: samples were stored at 4oC. Subsamples were dried

at 65oC and sent to Israel. The samples were ground and sieved to <1mm.

Compost from Israel: the ISBS sample was obtained from a windrow compost

process. The sample was stored at 4oC. Subsamples were dried at 65oC, ground and

sieved to <5mm. This compost was analyzed using the same procedure at the same

time as the US based composts, as part of a series of samples. This sample is part of a

series that as been subjected to an in-depth investigation of which a large set of results

were published in a series of publications (Zmora-Nahum et al., 2007; Danon et al.,

2007; Zmora-Nahum et al., 2008a; Danon et al., 2008; Zmora-Nahum et al., 2008b).

Analysis: ash content was determined by combustion at 400oC for 8 hours.

Organic matter (OM) was determined by subtraction of the ash content. CaCO3

content was measured using a calcimeter.

7


Elemental analysis (C, H, N, S) was performed on samples using a Flash AE-

1112 Elemental Analyzer (Thermo Instruments, Delft, The Netherlands). Oxygen was

calculated by difference between the ash free OM and the sum of C+H+N+S.

Water extracts were prepared by shaking dry compost with deionized water in a

1:10 compost:water weight ratio for 2 hours, at room temperature. The suspension

was then centrifuged at 10,000 g for 30 min, filtered through Whatman GFI paper

then subsequently through a 0.45 µm membrane filter (Supor-450, Gelman Sciences).

The concentration of DOC was determined, after acidification to pH 5, on a Total

Carbon Analyzer, TOC-VCPH, Shimadzu, Tokyo, Japan. DOC concentration was

converted to bulk compost basis by dividing by a factor of 10.

Electrical conductivity (EC) of the extract at 25°C was measured using a

Radiometer, CDM83 conductivity meter.

pH was measured using a Metrom electrode and a Radiometer pH-meter.

Nitrate ion concentration in the extract was measured using a Radiometer ISE-

K-NO3 electrode.

Ammonium concentration in the extract was measured using a Tecator

Automatic Kjeldahl System.

Bicarbonate was measured using the potentiometric titration method and

employing the Gran Plot for data analysis.

UV-VIS absorption was measured using a Hewlett Packard 8452A Diode Array

spectrophotometer. From the UV-VIS spectra one may derive: (i) the Specific UV

Absorption (SUVA; at 254 nm) which is an indicator of aromaticity of the DOC,

which increases during composting; (ii) specific absorption peaks indicating the

presence at high levels of lignins and proteins (or pollutans); and (iii) the spectrum

facilitates calculations of the E4/E6 ratio (light absorption at 465 nm divided by that at

665 nm). This ratio indicates molecular dimensions of the DOC and usually decreases

with composting (reduction of concentrations of small molecules).

DRIFT spectra of the bulk composts were obtained for a wave number range of

4000-400 cm-1 on a Nicolet 550 Magana-IR spectrometer (Nicolet Instruments

Corporation, WI). Finely ground samples were re-dried overnight at 65°C. Samples

(17 mg) were intimately mixed with 80 mg of dry KBr, and then mixed with

additional KBr to a final weight of 350 mg. The powder was dried for an additional

night before reading the IR diffuse reflectance (DRIFT). Twenty scans were collected

per spectrum. The spectra were baseline corrected and analyzed using the OMNIC

8


software (Nicolet Instruments Corp., Madison, WI). For unity of presentation and

analysis, spectra were baseline corrected using 4 reference points at 4000, 2000, 860

and 400 cm-1 and normalized by dividing all spectral points by the value at 2950 cm-1.

These spectra are used to evaluate the composition of the OM in composts. With

composting aliphaticity decreases (breakdown of carbohydrates) and aromaticity

increases indicating stability.

The procedure to obtain the concentration of micro-elements was 500 mg

batches of the sludge samples which were digested in 5 ml volumes of aqua regia in

Teflon vessels using a Milestone Ethos Microwave Sample Digestion System. The

volume was made up to 25 ml with deionized water. Element concentrations in the

clear solutions were measured by an End-On-Plasma ICP/AES model ‘ARCOS’

produced by Spectro GMBH, Germany. The measurements were calibrated with

standards for ICP from Merck.

In general, all the measurements were conducted on triplicate sub-samples.

3. Results 3.1 Organic matter and ash content

Percentage organic matter (OM) is presented in Table 2 and Figure 1. The values

varied between 30% to 56%. The ash percentage varied between 44% to 70%. The

"Bowling Green" compost exhibited a remarkably low OM content and consequently

high ash content. The other composts consisted of OM and ash levels within the

requirement of sewage sludge composts. Since the Israeli standard according to

compost classifications is 25-35% OM (% dry weight), all composts meet this

requirement.

Table 2: Average organic matter content of the composts Material Organic Matter content [%] Ash content [%] Delphos belt cake @ 26.89.TS 10/2/08 41.80±0.76 58.20±0.76 Yorkville 25.29.TS before drying 10/6/10 56.05±0.03 43.95±0.03 Bowling Green centrifuge 40% TC 10/2/08 30.01±0.03 69.99±0.03 ISBS 11 4/2/06 46.72±0.05 53.29±0.05

9


Organic Matter

0

10

20

30

40

50

60

Delphos belt cake @26.89.TS 10/2/08

Yorkville 25.29.TS beforedrying 10/6/10

Bowling Green centrifuge40% TC 10/2/08

ISBS 11 4/2/06

Material

Org

anic

mat

ter [

%]

(A)

Ash

0

10

20

30

40

50

60

70

80




ISBS 11 4/2/06

Material

Ash

[%]

(B)

Figure 1: Organic matter (A) and ash content (B) of the composts (standard deviations are too small to be seen).

3.2 Composts extract composition

3.2.1 Dissolved Organic Carbon: the Dissolved Organic Carbon (DOC)

concentration in the water extract of the composts is presented in Table 3

and Figure 2. The DOC concentration decreases during the composting

process (Zmora-Nahum et al., 2005). The rate of decrease depends on the

process and on the source material. Composting biosolids takes a longer

time to reach maturation and achieves values in the range of 2 g kg-1

10


(Zmora-Nahum et al., 2005). For the ease of units transformation we

would like to clarify that in order to obtain mg L-1 in a 1:10 extract from g

kg-1 units one should multiply the latter by a factor of 100. The final value

of different composts compared (biosolids, municipal solid waste and

separated cattle manure) was lower than 4 g kg-1 (or 400 mg L-1 in a 1:10

solid:water extract) and this value was proposed to be a threshold value

for mature composts. Levels of less than 2 g kg-1 were obtained towards

the end of the maturation process (Zmora-Nahum et al., 2005) .The DOC

concentration for the "Bowling Green" compost was lower than the

threshold value and the concentration of 2.78 g kg-1 indicates a compost at

the end of the maturation stage. The other three samples exhibited values

in the range of 6.7-7.7 g kg-1. These values are higher than the threshold

value and may indicate composts at a lower maturation degree, yet at a

sufficient maturity for field application.

Table 3: Average dissolved organic carbon concentration in the aqueous extract of the composts.

Material DOC [g kg-1] DOC [g L-1] Delphos belt cake @ 26.89.TS 10/2/08 6.65±0.31 739.11±26.44 Yorkville 25.29.TS before drying 10/6/10 7.32±0.11 821.97±10.82 Bowling Green centrifuge 40% TC 10/2/08 2.78±0.02 310.95±1.02 ISBS 11 4/2/06 7.72±0.31 886.12±9.88

Dissolved Organic Carbon

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00




ISBS 11 4/2/06

Material

Dis

solv

ed O

rgan

ic C

arbo

n [g

kg-1

]

Figure 2: Dissolved organic carbon concentration in the aqueous extract of the

composts.

11


3.2.2 E4/E6 ratio: the values of the E4/E6 ratio for the DOC in the water extract

of the composts are presented in Table 4 and Figure 3. The higher the

value, the smaller the size of the OM molecules (Chen et al., 1977). At

early stages of OM degradation, the extracts contain high concentrations

of non-absorbing, low molecular weight molecules such short chain

aliphatic acids, sugars and amino compounds. As composting progresses,

these molecules degrade and the extracts consist mostly of partially

humified compounds. According to the results, the "Bowling Green"

compost and our reference material (ISBS 11) exhibit lower ratio values

and therefore, higher molecular weight molecules in the DOC. In contrast,

in the Delphos and Yorkville compost extracts, the values are high,

indicating lower molecular weight molecules apparently indicating they

are at an early stage of the maturation process. It should be noted that the

ISBS 11 originates from sewage sludge blended with green (yard) waste

which continuously supply as a source of C and energy, which are the

source of DOC even though the material is highly matured. The lower

level of the E4/E6 ratio measured for the "Bowling Green" compost

resulted from the addition of bentonite during the thickening process

which removes low molecular weight from the solution.

Table 4: E4/E6 ratio of the dissolved organic carbon in the aqueous extract of the composts.

Material E4/E6

Delphos belt cake @ 26.89.TS 10/2/08 23.95±1.59 Yorkville 25.29.TS before drying 10/6/10 26.79±1.19 Bowling Green centrifuge 40% TC 10/2/08 11.90±2.24 ISBS 11 4/2/06 13.01±0.08

12


E4/E6

0

5

10

15

20

25

30




ISBS 11 4/2/06

Material

E4/E

6

Figure 3: E4/E6 ratio of the dissolved organic carbon in the aqueous extract of the composts.

3.2.3 Specific UV Absorbance (SUVA): the value calculated by dividing the

absorbance at 254 nm by the DOC concentration, is indicative of the

aromaticity and concentration of humic substances in Dissolved Organic

Matter (DOM) (Weishaar et al., 2003). As composts mature the

concentration of the easily degradable components decreases and that of

the recalcitrant, aromatic components is expected to increase. The values

of the SUVA for the DOC in the water extract of the composts are

presented in Table 5 and Figure 4. The value for the "Bowling Green"

(0.09x10-2 L mg-1) is relatively low compared to the other composts

(1.25x10-2-2.04x10-2 L mg-1). The results for the "Bowling Green"

compost do not seem to be in line with the assumptions presented because

some additions to the source materials e.g. flocculation materials. The

values for the Delphos and Yorkville products are within the line of

compost at the middle stage of stabilization, whereas that of the ISBS is

affected by aromatic components derived from lignin released from

woody yard products.

13


Table 5: SUVA values of the dissolved organic carbon in the aqueous extract of the composts.

Material Specific UV absorbance (SUVA) [L mg-1] Delphos belt cake @ 26.89.TS 10/2/08 1.41x10-2±8.29x10-4

Yorkville 25.29.TS before drying 10/6/08 1.25x10-2±8.20x10-4

Bowling Green centrifuge 40% TC 10/2/08 0.09x10-2±7.61x10-4

ISBS 11 4/2/06 2.04x10-2±7.55x10-4

SUVA

0

0.005

0.01

0.015

0.02

0.025




ISBS 11 4/2/06

Material

Spec

ific

UV

abso

rban

ce-S

UVA

[l m

g-1]

Figure 4: SUVA values of the dissolved organic carbon in the aqueous extract of the composts.

3.2.4 UV/VIS spectra: the spectra of the DOC of the aqueous extract,

measured in the range 200-800 nm, are presented in Figures 5 to 8. No

specific peak can be observed in the spectra of all the composts. A plateau

has been observed in the range 260-380 nm. This plateau is broader for

the "ISBS" compost and its width decreases in the "Delphos" and

"Yorkville" composts. In the "Bowling Green" material only a small

shoulder can be observed at 256 nm. Also in these measurements, the

"Bowling Green" compost differs from the other 3 composts. This plateau

indicates a high level of lignin derived products (ISBS) and those of

protein origin.

14


Delphos belt cake @ 26.89.TS 10/2/08-1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce

Figure 5: Spectra UV/VIS of the dissolved organic carbon in the aqueous extract of

the "Delphos" compost.

15


Yorkville 25.29.TS before drying 10/6/08-1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


the "Yorkville" compost.

16


Bowling Green centrifuge 40% TC 10/2/08-1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


the "Bowling Green" compost.

17


ISBS 11 4/2/06-1

0

1

2

3

4

5

6

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce

ISBS 11 4/2/06 -2

0

1

2

3

4

5

6

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce

ISBS 11 4/2/06-3

0

1

2

3

4

5

6

200 300 400 500 600 700 800Wavelength (nm)

Abs

orba

nce


the "ISBS 11" compost.

18


3.2.5 pH, electrical conductivity (EC) and N-NO3: the pH values, EC and N-

NO3 concentration in the water extract of the composts are presented in

Table 6. The pH values were in the range 6.78-7.23. In a comparative

study of composts varying in source materials, the pH was found to range

from 5.27 to 8.36 (Zmora-Nahum et al., 2007). A value of pH in the range

6.5 to 7 is considered an indicator of a proper and full composting process

(Alexander, 1977; de Bertoldi et al., 1987), but certainly pH values should

not be considered as a "fine tune" indicator.

EC values ranged from 0.78 to 3.35 dS m-1. All the EC values are low and

suitable for agriculture requirements (below 10 dS m-1- S-100 Wisconsin;

Israel-IS-801). In general, low EC values are characteristics of composts

processed in open windrows where salts are leached and removed with the

drainage water. The low values measured for the three composts

processed in the closed system result from the process (liquid

composting) and are valuable and beneficial characteristics for

agricultural application.

The N-NO3 concentration increases in the last steps of the composting

process as a result of aerobic conditions and achieve values higher than

the very low concentration in raw sludge (Inbar et al., 1993; Bernal et al.,

1998b; Garcia et al., 1991). The concentration measured in the composts

from the US ranged from 28.5-121.5 mg L-1, whereas N-NO3 in the ISBS

sample was 98 mg L-1.

Table 6: pH, electrical conductivity (EC) and nitrogen-nitrate concentration in the aqueous extract of the composts.

pH EC N-NO3 Parameter Material dS m-1 mg L-1

Delphos belt cake @ 26.89.TS 10/2/08 6.98±0.02 1.21±0.01 28.5±22.8 Yorkville 25.29.TS before drying 10/6/10 6.82±0.01 1.87±0.02 121.5±36.9 Bowling Green centrifuge 40% TC 10/2/08 7.23±0.04 0.78±0.03 73.7±10.7 ISBS 11 4/2/06 6.78±0.01 3.35±0.03 98.0±2.0

3.2.6 N-NH4 concentration: the N-NH4 concentrations in the water extract of

the composts are presented in Table 7. The N-NH4 concentrations

decrease in the last stages of the composting as a result of aerobic

conditions and a nitrification process. The concentrations measured in the

19


composts tested were in the range 178.4-267.3 mg L-1 N-NH4. These

values are characteristics of early maturation stages.

Table 7: N-NH4 concentration in the aqueous extract of the composts. Material N-NH4 concentration [mg L-1] Delphos belt cake @ 26.89.TS 10/2/08 81.4±3.4 Yorkville 25.29.TS before drying 10/6/10 170.3±0.7 Bowling Green centrifuge 40% TC 10/2/08 82.7±4. 9 ISBS 11 4/2/06 160.4±3.2

3.2.7 Bicarbonate: the bicarbonate (HCO3-) concentrations in water extracts of

the composts are presented in Table 8. The concentrations are in the range

of 41.3 to 235.7 mg L-1. Bicarbonate concentrations decrease during

composting due to a decrease in respiration. The lowest value was

observed in the "Bowling Green" compost in agreement with the lower

DOC concentration and higher maturity level.

Table 8: Bicarbonate concentration in the aqueous extract of the composts. Material Bicarbonate [mg L-1] Delphos belt cake @ 26.89.TS 10/2/08 109.0±18.4 Yorkville 25.29.TS before drying 10/6/08 135.0±24.6 Bowling Green centrifuge 40% TC 10/2/08 41.3±7.5 ISBS 11 4/2/06 235.7±10.6

3.3 Micro-elements: Micro-elements concentrations in the composts are presented in

Table 9. Differences between were obtained among the various materials but, as

expected, due to different source materials. In general concentrations of metals

are related to their concentration in the wastewater. The relative proportion of

industrial waste entering the sewer system relative to the domestic wastewater

determines the final micro-element concentration in the compost. The standards

for heavy metal concentration in Israel, the European Union and USA-EPA are

presented in Table 10.

Cadmium (Cd) concentrations are in the range 0.5-1.1 mg kg-1, which are lower

than the strictest regulation in the EU (10 mg kg-1). Chrome (Cr) concentrations

are in the range 40.1-88.6 mg kg-1, which are lower than the strictest regulation in

Israel (400 mg kg-1). Copper (Cu) concentrations are in the range 238-1094 mg

kg-1: the three "ThermAer" composts exhibit higher concentrations than those of

the strictest regulation (Israel; 600 mg kg-1), but yet lower than those of the EU

and EPA standards. Mercury (Hg) concentrations are in the range 1.3-3.9 mg kg-

1, which are lower than the strictest regulation (Israel; 5 mg kg-1). Nickel (Ni)

20


concentrations are in the range 23.8-42.2 mg kg-1, which are lower than the

regulation in Israel (90 mg kg-1). Lead (Pb) concentrations are in the range 24.1-

72.2 mg kg-1, which are lower than the regulation in Israel (200 mg kg-1). Zinc

(Zn) concentrations are in the range 610.8-1187.1 mg kg-1, which are lower than

the strict regulation in Israel (2500 mg kg-1).

Boron (B) concentration limit in the Israeli Compost Standard is 200 mg kg-1 dry

matter. The concentrations found in the present work are lower and within a

range of 34.2-67.7 mg kg-1.

Table 9: micro-elements concentration in the composts. Delphos Yorkville Bowling

Green ISBS 11

Sample

Element Concentration [mg kg-1] B 67.7 65.4 34.2 58.5

Cd 1.1 0.8 0.5 0.9 Cr 78.9 58.4 88.6 40.1 Cu 887 1094 526 238 Hg 1.3 3.6 1.7 3.9 Ni 40.7 30.9 42.2 23.8 Pb 58.9 24.1 72.2 47.3 Zn 610.8 883.1 1187.1 817.6

Table 10: Biosolids quality standard - heavy metals standard. USA European Union Israel Country EPA Proposed

Directive Directive

86/278/EEC Israel Water Regulations

Element Concentration [mg kg-1] Cd 85 10 20-40 20 Cr 3000 ------ -------- 400 Cu 4300 1000 1000-1750 600 Hg 57 10 16-25 5 Ni 420 300 300-400 90 Pb 840 ------ 750-1200 200 Zn 7500 2500 2500-4000 2500

3.4 Elemental Analysis: the elemental (C, N, H, S, O) composition of the composts

is presented in Table 11 and Figure 9. Nitrogen content ranges from 5.93 to

8.34%. The N concentration is lower in the ISBS compost than in composts from

the US. The lower N content in the ISBS can be explained by the addition of yard

waste prior to composting which is essential in windrow composting of biosolids.

All the tested materials exhibited higher N content in comparison with a series of

compost from commercial composting facilities in Europe (0.67-3.79% N)

21


(Zmora-Nahum et al., 2007). This higher content is a positive in relation to the

nutrient value of the applied compost to agriculture and landscape crops.

Carbon content ranged from 48.5 to 54.7%. There are no significant differences

in the C and S content among the tested materials. The C/N ratio (Table 12) in the

composts tested (6.2-8.4) is typical of biosolids compost due to the sludge OM

composition. When yard waste is added as in the case of ISBS this low ratio is an

indicator of maturity (Markovitch, 2004; Zmora-Nahum et al., 2005) The H/C

atomic ratio in the composts (1.69-2.11) is higher than the values observed in

humic and fulvic acids (1.1-1.2) and of mature biosolid composts prepared with

yard waste (Markovitch, 2004) indicating that the materials are more aliphatic in

the composts than in humic substances.

The low SUVA values observed in the aqueous extracts of the composts verify

this assumption. The "ThermAer" composts present higher H/C ratios than the

windrow compost (ISBS 11). Similarly, the O/H ratio in the biosolid composts

(1.70-2.23) is by far higher than the ratio in humic substances (0.30-0.45)

indicating that the OM in the compost is in a higher oxidation stage.

Table 11: Elemental composition of the composts (concentration in ash free dry matter).

Material N [%] C [%] H [%] S [%] O [%] Delphos belt cake @ 26.89.TS 10/2/08 7.60±0.42 48.50±3.12 8.36±0.61 1.19±0.07 34.36±4.19 Yorkville 25.29.TS before drying 10/6/08 8.34±0.19 51.69±0.94 8.19±0.26 1.18±0.06 30.60±1.34 Bowling Green centrifuge 40% TC 10/2/08 7.15±0.14 54.69±0.90 9.60±0.13 1.39±0.07 27.17±0.93 ISBS 11 4/2/06 5.93±0.47 50.03±4.86 7.04±0.66 1.30±0.16 35.71±6.13

Table 12: Atomic ratios derived from the elemental composition of the composts.

Material C/N H/C O/H Delphos belt cake @ 26.89.TS 10/2/08 6.38 2.07 2.15 Yorkville 25.29.TS before drying 10/6/08 6.20 1.90 1.91 Bowling Green centrifuge 40% TC 10/2/08 7.66 2.11 1.70 ISBS 11 4/2/06 8.43 1.69 2.23

22


Elemental Analysis

7.6 8.4

1.2

8.3

51.7

8.2

1.2

30.6

7.1

54.7

9.6

1.4

27.2

5.9 7.0

1.3

34.4

48.5

50.0

35.7

0

10

20

30

40

50

60

N C H S OElement

Elem

ent c

once

ntra

tion

[%]

Delphos belt cake @ 26.89.TS 10/2/08

Yorkville 25.29.TS before drying 10/6/08

Bowling Green centrifuge 40% TC 10/2/08

ISBS 11 4/2/06

(A)

C/N ratio

0

1

2

3

4

5

6

7

8

9




ISBS 11 4/2/06

C/N

ratio

(B)

Figure 9: Elemental analysis (A) and C/N ratio of the composts (B).

3.5 CaCO3 content: the CaCO3 content of the composts is presented in Table 13.

The three samples obtained from the "ThermAer" process contain negligible

concentrations of CaCO3. In contrast, the compost from Israel contains a

relatively high concentration. These differences can be explained by the

quality of the fresh water (high hardness) in Israel and consequently the

chemical properties of the wastewater that cause increased precipitation of

CaCO3 during the treatment and sludge separation. An additional explanation

23


stems from the use of yard waste in windrows. This material is richer in

CaCO3 than biosolids.

Table 13: CaCO3 concentration in the composts.

Material CaCO3 [%] Delphos belt cake @ 26.89.TS 10/2/08 0.10±0.17 Yorkville 25.29.TS before drying 10/6/10 0.14±0.25 Bowling Green centrifuge 40% TC 10/2/08 0.00±0.00 ISBS 11 4/2/06 3.64±0.10

3.6 Diffuse Reflectance Infrared Fourier Transform (DRIFT)

IR spectra of composts typically exhibit the following peaks: a broad band at

3300-3400 cm-1 (H-bonded OH groups), sharp peaks at 2925-2930 and 2850 cm-1

(aliphatic C-H stretch), a peak or shoulder at 1715 cm-1 (carbonyl or carboxyl

C=O), a broad peak which may encompass the amide C=N at 1640-1650 cm-1

and the aromatic C=C at 1620 cm-1, a peak at 1540-1560 cm-1 (amide II bonds), a

peak at 1450-1460 cm-1 (aliphatic C-H or aromatic ring stretch), a strong peak

either at 1100-1110 or 1030 cm-1 (C-O stretch of polysaccharides) (Inbar et al.,

1989, 1991; Chefetz et al., 1998). The composts tested in the current research

present similar spectra (Figure 11a to d): two relative high aliphatic peaks (2925

and 1460 cm-1), a high aromatic or amide peak at 1650-1660 cm-1, a peak at 1540

cm-1 (amide II) and a very pronounced peak at 1030 cm-1. The prominence of the

aliphatic peaks are indicative of a relatively low level of degradation, namely, the

OM is only partially degraded. This high peak may either be -OH stretch of

polysaccharide, or may result from phosphate or silicate minerals (Outmane et

al., 2000). The exceptional height of this peak relative to the other peaks in the

spectra, indicates that in the case of biosolids which do not contain minerals this

peak results from polysaccharides. Inbar et al. (1989) found that from day 18 of

manure composting the relative intensity of the 1655, 1425 and 1060 cm-1

remained at approximately 1:0.9:1.3. In the present report, the ratio of aliphatic to

aromatic or amide peak (1425/1665) is similar in all the samples (Table 10), but

lower than the ratio in manure composts, indicating a decrease of the aliphatic

peak relative to the aromatic or amide peak. These findings are in accordance

with the high presence of straw in manure (Inbar et al., 1989), in contrast to the

biosolid composts which are richer in proteins. The ratio of polysaccharide to

aliphatic peak (1060/2925) is distinctive of different groups of composts and is

24


related to the OM content (Zmora-Nahum et al., 2007). For the biosolids (Table

14) all the values are related to a high OM content.

Table 14: Ratio of Diffuse Reflectance Infrared Fourier Transform (DRIFT) peaks. Peak ratio Material 1655/1655 1425/1655 1060/1655 1060/2925 Delphos belt cake @ 26.89.TS 10/2/08 1 0.6 1.3 1.6 Yorkville 25.29.TS before drying 10/6/10 1 0.7 0.9 0.8 Bowling Green centrifuge 40% TC 10/2/08 1 0.5 1.6 1.5 ISBS 11 4/2/06 1 0.7 1.0 0.8

25


Delphos belt cake @ 26.89.TS 10/2/08

1456

1545

1032

1660

2927

3294

80012001600200024002800320036004000

Wavenumber [cm-1]

(A)

Yorkville 25.29.TS before drying 10/6/08

1080

14561543

1662

2933

3305

80012001600200024002800320036004000

Wavenumber [cm-1]

(B)

26


Bowling Green centrifuge 40% TC 10/2/08

1041

16622929

3311

80012001600200024002800320036004000

Wavenumber [cm-1]

(C)

ISBS 11 4/2/06

1049

1456

1653

2939

3302

80012001600200024002800320036004000

Wavenumber [cm-1]

(D)

Figure 11: Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectra of (A)

Delphos biosolid compost; (B) Yorkville biosolid compost; (C) Bowling Green biosolid compost; and (D) ISBS 11 biosolid compost.

27


Conclusions

In conclusion, the "ThermAer" composts are very similar to biosolid compost

processed at length to stability by an open windrows system. The composts are in

an early stage of maturation, yet they are very stable and odorless. In addition to

the chemical traits indicating stability they are stored for periods of months

without the development of offensive odors. As well they do not emit heat. These

two facts provide additional evidence for their stability which is an important

asset to both the producers and potential users. The high nitrogen content in the

biosolid composts represents an agronomic advantage along with low ash and

CaCO3 content. All composts meet the heavy metals concentration requirements.

Thus the "ThermAer" process which is short and efficient stands out as a very

attractive option to municipalities, while ensuring beneficial agricultural and

landscaping utilization.

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