Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 7)

Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 7)

This is the seventh quarterly progress report (date: 28th January 2015) in this two-year project (starting date: 12th August 2013). The scope of the 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) assays on the sludge samples, and (3) characterization of centrate of the BMP reactors. The BMP assays aim to provide information on anaerobic biodegradability of the sorption-enhanced primary sludge in comparison with the conventional activated sludge. The results attained may serve as a baseline to design anaerobic digestion system and to optimize biogas production. The analysis of sludge samples collected from Jurong WRP has been completed in the first part of the project (Aug 2013 to March 2014). The present work thus concentrates on the samples from Ulu Pandan WRP.

In Ulu Pandan WRP, the novel biosorption method to enhance primary sludge is investigated in a pilot-scale AB system called integrated validation plant (IVP). The system primarily consists of a biosorption pretreatment tank followed by a primary clarifier in the A-stage and a membrane bioreactor (MBR) in the B-stage. Sludge samples for this study are withdrawn from the clarifier and MBR, respectively. A proposed schematic process of the IVP is presented in Figure 1. The influent to IVP consists of incoming wastewaters and dewatered sludge from Ulu Pandan WRP’s solid treatment facilities. A total of 93 samples (comprising 46 samples from the clarifier and 47 samples from the MBR) have been collected to date.

Figures 2 and 3 show the profiles of solids content (TS, VS, TSS, and VSS) and COD concentration of both clarifier and MBR samples. It can be seen that trends in volatile solids content are reflected in total COD concentration profiles, with higher fluctuation obtained from the clarifier samples as compared to the MBR samples. Similarly, the VS/TS ratio of the clarifier samples is notably more dynamic, ranging from 0.36-0.84 while a relatively constant range (0.65-0.80) was obtained from the MBR samples. Such dynamics is likely attributed to Fe precipitates from the addition of ferric chloride to Ulu Pandan WRP’s digester slurry in order to suppress H2S.
Figure 4 presents the comparison of calorific value and BMP assays’ biodegradability results of both sludge samples. 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: 14,000-20,000 J/g TS; biodegradability: 120-400 mL CH4/g VS). Indeed, these two properties are closely linked to the solids content, i.e. fixed and volatile solids composition. It is also noteworthy that clarifier samples demonstrated higher biodegradability than the MBR samples at all times, as shown in Figure 5.

At the end of the BMP assay, the centrate was extracted from the batch 300 mL reactors in the laboratory (AMPTS) and subsequently analyzed for its C-N-P content. The typical ranges for SCOD, TN, and TP are 50-200 mg COD/L, 300-600 mg NH4+-N/L, and 75-250 mg PO43--P, respectively.

Unpublished report.