Elsevier

Science of The Total Environment

Volume 443, 15 January 2013, Pages 877-886
Science of The Total Environment

Anaerobic co-digestion of source segregated brown water (feces-without-urine) and food waste: For Singapore context

https://doi.org/10.1016/j.scitotenv.2012.11.016Get rights and content

Abstract

The objective of this study was to evaluate the feasibility of anaerobic co-digestion of brown water (BW) [feces-without-urine] and food waste (FW) in decentralized, source-separation-based sanitation concept. An effort has been made to separate the yellow water (urine) and brown water from the source (using no-mix toilet) primarily to facilitate further treatment, resource recovery and utilization. Batch assay analytical results indicated that anaerobic co-digestion [BW + FW] showed higher methane yield (0.54–0.59 L CH4/gVSadded) than BW or FW as a sole substrate. Anaerobic co-digestion was performed in the semi-continuously fed laboratory scale reactors viz. two-phase continuous stirred-tank reactor (CSTR) and single-stage sequencing-batch operational mode reactor (SeqBR). Initial 120 d of operation shows that SeqBR performed better in terms of organic matter removal and maximum methane production. At steady-state, CODs, CODt, VS removals of 92.0 ± 3.0, 76.7 ± 5.1 and 75.7 ± 6.6% were achieved for SeqBR at 16 d HRT, respectively. This corresponds to an OLR of 2–3 gCOD/L d and methane yield of about 0.41 L CH4/gVSadded. Good buffering capacity did not lead to accumulation of VFA, showing better process stability of SeqBR at higher loading rates. The positive findings show the great potential of applying anaerobic co-digestion of BW + FW for energy production and waste management. In addition, daily flush water consumption is reduced up to 80%. Decentralized, source-separation-based sanitation concept is expected to provide a practical solution for those countries experiencing rapid urbanization and water shortage issues, for instance Singapore.

Highlights

► Source separation of organic waste/wastewater streams on household level was done. ► Brown water (BW) was collected from a specially designed no-mix toilet. ► BW and food waste codigestion proved as a potential substrate for biogas production. ► A distinct improvement in methane yield was observed. ► This concept is vital for countries facing rapid urbanization and water shortage.

Introduction

The decentralized treatment of municipal wastewater based on separation between gray and black water, and even between brown water (BW) [feces-without-urine] and yellow water (YW) [urine], represents a sustainable and future solution for waste (water) treatment (Elmitwalli et al., 2006). The separation of different wastewater streams and their treatments with the aim of energy production and nutrient reuse was demonstrated in the year 2000 within a housing estate for 350 to 400 inhabitants in the pilot project Flintenbreite in Luebeck, Germany (Wendland and Oldenburg, 2003, Wendland et al., 2007). The concept comprises vacuum toilets with subsequent pasteurization and anaerobic digestion (AD) of black water together with kitchen waste in a semi-centralized biogas plant and finally recycling of the digested anaerobic effluent in agriculture. A few other researchers have also studied the co-digestion of black water and kitchen refuse in various anaerobic systems (Kujawa-Roeleveld et al., 2003, Kujawa-Roeleveld et al., 2005, Kujawa-Roeleveld et al., 2006, Elmitwalli et al., 2006, Wendland et al., 2007). In addition, these researchers have successfully demonstrated the feasibility of treating human waste in decentralized sanitation systems. However, source separation between feces and urine, and its subsequent resource recovery approach is limited. Besides, such research in the urban context has been scarce.

On the other hand, an alarming aspect worldwide is the depletion of non-renewable energy sources. Natural resources are not efficiently used by human beings. According to an industrial ecology study (Deschenes and Chertow, 2004), only about 6% of material flows end up in making products and the majority of the remaining natural resources are considered as unusable waste in our industrial systems. If natural resources can be more efficiently used, the rapid depletion of resources can be mitigated and, at the same time, waste management problems can be resolved.

An innovative source separating toilet can separate YW and BW to facilitate further treatment, resource recovery and utilization. The collected YW can be properly treated for nutrient (nitrogen, phosphorus) recovery in order to produce fertilizer and soil amendments (Sundin et al., 1999). This can be another source of revenue. Alternatively, this paper presents the potential alternative of using source separated BW as a feed source for bio-energy production. AD system symbolizes a sustainable and low-cost technology for waste (water) treatment. Therefore, it is profitable to apply AD within decentralized sanitation.

According to the waste statistics from Singapore's National Environment Agency (NEA), the annual generation of food waste (FW) was 542,700 tonnes in 2006 and reached about 640,500 tonnes in 2010, which is around 10% of the total waste output in Singapore. However, only 16% of FW was recycled and the rest of FW was sent to waste-to-energy incineration plants. Organic waste presents more difficulties in recycling of FW because of the associated bad smell and contamination caused by the organics. Singapore, a small island, justifiably finds this a big issue. Therefore, the recycling rate of FW remains very low in Singapore, about 7%, over the last 20 years. The recent NEA report shows that the recycling rate for FW has dropped from 16% in 2010 to 10% in 2011 (Singapore waste statistics, 2011). It is likely due to the cease of operation of a giant waste management company in Singapore, last year, which was recycling FW into biogas and compost. There is currently no news of the setting up of new food waste recycling plants, nor is there any food waste reduction campaign. This evidently shows that there is an alarming need for the FW treatment and management in Singapore. Hence in this study, household FW management and treatment has been given adequate priority. In addition to BW, the AD system can also digest kitchen organic-wastes, which will also improve the potential of the utilization of biogas produced from the AD system, as kitchen organic-wastes have a high-organic content.

The aim of this paper was to evaluate the technical feasibility of anaerobic co-digestion of brown water (BW) [feces without urine] and household food waste (FW) and to identify the key operating conditions governing the process performance. Special focus was put on the determination of (i) biomethane potential of co-digestion of BW and FW in a batch assay; (ii) anaerobic biodegradability of the waste mixture (BW + FW); (iii) laboratory scale two-phase continuous stirred-tank reactor (CSTR) and single-stage sequencing-batch operational mode reactor (SeqBR) performances and process efficiencies; and (iv) microbial population in the various anaerobic system configurations.

Section snippets

Feedstock and inoculum sources

Food waste (FW) refers to leftover food. FW was collected once a week from one of the canteens at NTU campus, where the majority of the waste came from Chinese, Indian, Indonesian and Malay food stalls. It was a mixture of meat, rice, noodles, vegetables and salad. After bones and non-food materials were removed, the FW was then crushed by a kitchen blender to promote homogeneity of the substrate as well as disintegration of particulate organics. The blended FW was then mixed well, and stored

Operational scheme and the characteristics of feed

The feeding mixture of BW and FW was prepared bi-weekly and subsequently fed to the reactors or stored at 4 °C until needed for feeding. The reactors were fed with BW + FW mixture every day in a semi continuous mode. A life cycle assessment (LCA) report by Remy (2010) suggests 160 g of FW per person per day as a good reference. Singaporeans seldom cook at home; therefore, an estimation of 150 g is used for household FW in this study. BW amounts to about 2 L (feces + flushing water). Based on this

Conclusions

This study demonstrated that anaerobic co-digestion of BW and FW proved to be a potential substrate for biogas production. Anaerobic co-digestion showed higher methane yield than BW and FW in non-mixture conditions. SeqBR allowed a balanced conversion of organics to CH4 at an OLR of 2–3 gCOD/L d that corresponds to a VS removal of about 75%. BW provided a sufficient buffering capacity to the FW digestion. We believe “decentralized and source-separation-based sanitation concepts” can eventually be

Acknowledgments

The authors are grateful to the National Research Foundation (NRF), Singapore for financial support (NRF-CRP5-2009-02). We appreciate Mr. B. Wang, Mr. Bernard Ng and Mr. G.W.H. Chia for their helping hands and cooperation in the experimental work. We are grateful to Prof. Rainer Stegmann and Prof. C.S. Lee for stimulating discussions in this project. We are thankful to R3C/NTU family for their contributions to this research program.

References (27)

  • P.J. Deschenes et al.

    An island approach to industrial ecology: towards sustainability in the island context

    J Environ Plann Manag

    (2004)
  • T.A. Elmitwalli et al.

    Anaerobic biodegradability and digestion in accumulation systems for concentrated black water and kitchen organic-waste

    Water Sci Technol

    (2006)
  • H.W. Kim et al.

    Response surface optimization of substrates for thermophilic anaerobic codigestion of sewage sludge and food waste

    J Air Waste Manag Assoc

    (2007)
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