Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 8)

Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 8)

8th Quarterly Progress Report.

Date of Progress Report : 5th May 2015
Period of Research Project: 12th August 2013 – 11th August 2015.

Concerns on global warming and energy costs call for more effective approaches to improve the energy self-sufficiency of large centralized wastewater treatment plants (WWTPs). One of such approaches is to recover the potential energy available in raw municipal wastewaters. A well-structured strategy deploying a two-stage process, the so-called AB process, has been highly suggested for the implementation of energy-efficient WWTPs. The first stage is a treatment in an extremely high loaded biosorption stage (A-stage), which is subsequently followed by a treatment in low loaded biological stage (B-stage) ensuring removal of dissolved organics and ammonia. The entrapped organics (energy) during the A-stage could be recovered through biogas generation from the anaerobic digester. This project therefore aims to evaluate the feasibility of biosorption method as primary step (the A-stage’s mechanism) in the treatment of municipal wastewaters in Singapore. The scope of this research project includes (1) determination of solids content, COD concentration, and calorific value of sludge samples from pilot-plants located in Jurong and Ulu Pandan WRPs, (2) conducting biochemical methane potential (BMP) assay on the sludge samples, and (3) characterization of centrate of the BMP reactors. The BMP assay was performed to obtain anaerobic biodegradability of the sorption-enhanced primary sludge (the A-stage’s sludge) in comparison with the conventional activated sludge (the B-stage’s sludge). The obtained results may serve as preliminary information for designing anaerobic digestion system and optimizing biogas production. The analysis of sludge samples collected from Jurong WRP has been completed in the first part of the project (August 2013 to March 2014). The present work thus concentrates on the samples from the Integrated Validation Plant (IVP) located in Ulu Pandan WRP.

Figure 1 shows the proposed schematic of the IVP. The influent to IVP is a mixture of incoming wastewaters and dewatered sludge from Ulu Pandan WRP’s dewatering facilities. This mixture is held in Ulu Pandan WRP’s equalization tank. Sludge samples for this study are taken from the clarifier located at the downstream of biosorption pretreatment tank (the A-stage reactor) and MBR (the B-stage reactor). A total of 116 samples (comprising 58 A-stage samples, hereafter referred to as clarifier samples, and 58 B-stage samples, hereafter referred to as MBR samples) have been collected to date.

The profiles of solids content (TS, VS, TSS, and VSS) and COD concentration of both clarifier and MBR samples are shown in Figures 2 and 3, respectively. It can be seen that (1) the trends in organic content (VS and VSS) are reflected in total COD concentration profiles; and (2) higher fluctuation was observed from the clarifier samples as compared to the MBR samples. A more dynamic VS/TS ratio was also noted from the clarifier samples (0.36-0.84) in comparison with MBR samples (0.65-0.81). Such dynamics is likely attributable to the presence of Fe precipitates resulting from the addition of ferric chloride to Ulu Pandan WRP’s digester slurry for H2S suppression.

The comparison of calorific value and BMP assay’s biodegradability data of both sludge samples is presented in Figure 4. The calorific value was determined by bomb calorimetry while the BMP assay was conducted using automatic methane potential test system (AMPTS) conducted at 35°C. Greater variations in calorific and biodegradability values were observed from clarifier samples (calorific value: 10,000-22,000 J/g TS; biodegradability: 250-775 mL CH4/g VS) as compared to the respective properties of MBR samples (calorific value: 13,500-20,000 J/g TS; biodegradability: 120-450 mL CH4/g VS). These findings could be expected as the two properties are closely associated with the corresponding solids content, i.e. fixed and volatile solids composition. It is also noteworthy that clarifier samples demonstrated higher biodegradability than the respective MBR samples at all times, as presented in Figure 5.

After BMP assay, the digested liquor was extracted from the AMPTS reactors and the centrate was immediately analyzed for its C-N-P contents. The concentration ranges of SCOD, TN, and TP for both clarifier and MBR samples are typically 50-200 mg COD/L (average approx. 115 mg COD/L), 250-650 mg NH4+-N/L (average approx. 410 mg NH4+-N/L), and 50-250 mg PO43--P (average approx. 105 mg PO43--P), respectively.

Unpublished report.