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Municipal sewage sludge as co-substrate in anaerobicdigestion of vegetable waste and biogas yield

J V Thanikal1, M. TORRIJOS2, Philipe SOUSBIE2, S M Rizwan1, R. Senthil Kumar3, Hatem Yazidi1

Caledonian College of Engineering Muscat, Sultanate of Oman(A University college in Oman, affiliated to Glasgow Caledonian University, UK and Vellore Institute of Technology University, India)

12th Specialised Conference on Small Water and Wastewater System & 4th Specialised Conference on Resources Oriented Sanitation2-4 November, 2014, Muscat Oman

IntroductionCo-digestion is one of the advantages of anaerobic digestionprocess because several wastes having complimentarycharacteristics can be treated in a single process. The anaerobicco-digestion process, which can be defined as the simultaneoustreatment of two or more organic biodegradable waste streamsby anaerobic digestion offers great potential for the properdisposal of the organic fraction of solid waste coming fromsource or separate collection systems.


Proposed StudyThe aim of the proposed study was to analyse the mostquantitatively wasted vegetables in central vegetable market AlMawelah (local entity, Muscat, sultanate of Oman) and study theeffect of municipal sewage sludge as co-substrate on biogas yield.

The co-digestion of sewage sludge (SS) with other substrates, especiallyOFMSW, has a relatively long history, since it is the most popular co-digestion research subject. SS-OFMSW co-digestion is mainly affectedby the SS composition (primary, secondary or mixture) and differentkinds (quality) of OFMSW.


Materials and MethodsReactor and reactor operation conditionsThe experiments were carried out in double-walled glass reactors of 6-Leffective volume, maintained at 35 °C by a regulated water bath. Mixing inthe reactors was done by a system of magnetic stirring. The biogasproduction was measured on-line every 2 minutes by Milligascounter MGC-1flow meters (Ritter gas meters) fitted with a 4-20 mA output. The softwaresupplied by Ritter was used to log the gas output. The methane content inthe biogas was measured online using Blusens methane analyser and thedata was collected by software supplied by the manufacturer.

The reactor was seeded at a volatile suspended solids concentration (VSS)of around 6 g VSS/L with anaerobic sludge taken from an industrial-scaleanaerobic UASB reactor treating the effluents from a sugar refinery.


SubstrateThe primary and secondary sludge collected from domesticwastewater treatment plant were mixed to a proportion 40% and 60%respectively. The mixture was concentrated by settling where the totalsolids concentration was 1g/l. The type of vegetables, Potato, Carrotand Spinach, as vegetable substrate were collected based on thesurvey conducted at the Al Mawaleh central vegetable market, Muscat,Oman. The vegetables were shredded into small pieces ± 2 cm andused for feeding. Both the sewage sludge and vegetables were storedat -20 C.


Parameters Sewage Sludge Potato Carrot Spinach

Total Solids g/l 0.126 0.334 0.178 0.118Volatile Solids g/l 0.055 0.161 0.085 0.048

pH 7.2 ± 0.5 7.0 ± 0.5 7.3 ± 0.5 7.2 ± 0.5COD total /VS 2.1 2.2 1.8 0.9

C/N 11.42 22.35 25.62 12.45

Characteristics of Substrates

Substrate No. of batches Total Charge g VS HRT daysSewage sludge 2 3 5Carrot + Potato + Sludge 7 9 9 - 3Carrot + Potato + Sludge + Spinach 9 21 6 - 2Carrot + Potato + Spinach 6 24 4 - 3Carrot + Potato + Spinach 2 27 4 - 2Carrot + Potato + Spinach 5 36 1

Summary of Reactor Operation


Experimental Setup


Results and Discussions

OLR g VS/l.d 0.5 1.5 3.5 4.0 4.5 6.0

VS inlet g/l 3±0.5 9±0.5 21±0.5 24±0.5 27±0.5 36±0.5

VS outlet g/l 3.37±0.2 2.42±0.4 5.3±0.2 6.23±0.3 6.0±0.2 6.42±0.3

Percentage removal % 0.0 73.0 75.0 74.0 78.0 82.0

Biogas yield l/d 0.253 0.745 2.2 4.6 7.8 9.6

Bio Methane Poential(BMP) ml/g VS

134.67 248.17 222.5 244.33 299.8 283.1

pH 7.0±0.5 7.0±0.5 7.0±0.5 7.0±0.5 7.0±0.5 7.0±0.5

Digester performance


Snapshot of online accusation of data by Ritter gas flow meter showing each batch


Snapshot of online accusation of % volume of methane (top curves)


Biogas productionThe reactor was fed with different substrates varying the organicloading rate and also the HRT. The results of biogas production forsewage sludge, when used as a single substrate, were low(350ml/d), and also the biodegradation rate was slow. The sewagesludge was then co-digested with combinations of vegetables tostudy the biogas yield. The batch operated with potato as a singlesubstrate shown biogas production of 1.4 l/d. The addition ofvegetables along with the sewage sludge enhanced the biogasyield.The batches operated with total charge of 9g VS shown a biogasproduction increasing from 0.6 – 1.5 l/d. The biogas productionincreased almost to double the quantity when the total charge wasincreased to 18 g VS. Further addition of substrate (Spinach), with atotal charge 21g VS, did not have much influence of the biogasproduction.


vegetable substrates alone were fed to the reactors to see the effectof sewage sludge. The vegetable substrates (Potato, Carrot, Spinach)with a total charge of 27 – 36 g VS, with a HRT starting from 3 daysand ending with 1 day, shown a considerable increase in biogas yield3.5 - 5 l/d.

The batch studies shown that the sewage sludge did not have muchinfluence in biogas production and this may be due to i) the low VSsolid content ii) slow degradation, (H. Bouallagui et al.).

Higher degradation rate and biogas yield was shown at increasedOLR from 0.5 – 6.0 g VS/l (Fig.1).


It is very likely that the high degradation efficiency in the co-fermentation was due to an improved ratio of nutrients and betteravailability of the organic substances.

The COD accumulation in the reactor was not very high compared tothe number of days for which the reactor was operated. There wereminor variations which was almost constant during the last batch offeed


Conclusion

Anaerobic co-digestion of different vegetable substrates alongwith sewage sludge is found to be an interesting option.

Its benefits include improved balance of nutrients, synergisticeffect of microorganisms, increased load of biodegradableorganic matter and better biogas yield.

The addition of sewage sludge as a co-substrate did not havemuch effect on the yield of biogas.

The C/N ratio in the sewage sludge could keep a balance in theco-digestion system.

Sewage sludge of high VS content may be tried for morevariation in the effect of biogas yield.

The system proved to be stable at high OLR showing biogasyield.


AcknowledgementThe research work is a part of the funded research (2013-2015)from the “The Research Council of Oman” (TRC), Al Mawelahcentral vegetable market and Municipal wastewater treatmentplant, Samail, Muscat.

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