Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 4)

Analysis of samples and anaerobic biodegradability of sludges from Jurong and Ulu Pandan water reclamation plants (WRPs) (Report 4)

This is the fourth report (date: 6th May 2014) in this two-year project (starting date: 12th August 2013).
So far, we have analyzed 61 sludge samples from the pilot plant in Jurong WRP (57 from the clarifier and 4 from the biosorption column). We have analyzed TS, VS, TSS, VSS, TCOD and SCOD. The updated graphs are shown in the report in attachment. We still have about 15 samples left for the analysis of calorific value. This delay is due to the difficulty to analyze calorific value on some samples from JWRP. The vessel broke during the analysis of JWRP samples and had to be replaced. This may be due to the fact that these sludges are industrial and may contain salt or heavy metals (high inorganic content).

It can be seen that the TS and VS of the clarifier sludge varied widely in the range 0.5-5% or 5 to 50 g/L. The VS/TS ratio was very low in the range 24-55% presumably due to the industrial nature of the raw wastewater. Due to the variation in raw wastewater solid content, TCOD of the clarifier sludge varied between 3000 and 40,000 mg/L, while the SCOD was in the range 200 -1,800 mg/L. The SCOD/TCOD varied between 2 and 24%. Most of the COD was in particulate form. There was a clear correlation between the TS and the TCOD which showed that sludge with higher TS content had also a higher TCOD content. It should be noted that the sludge became more constant after December 2013 with a TS around 1-2%, VS lower than 1%, a TCOD lower than 20,000 mg/L and a SCOD lower than 1,000 mg/L. Various operating parameters were tested during the trial such as DO set point in the biosorption column (0, 0.2, 0.4, 0.5, 0.7 and 1 mg/L), Fe addition in the biosorption column (0, 5, 10 and 20 ppm) and return activated sludge (RAS – 0.21, 0.29 and 0.5 m3/hr), but these parameters did not seem to affect significantly the properties of the sludges tested.

The calorific value fluctuated around 10,000 J/g at the beginning of the project (Aug-Oct 2013), but increased to around 15,000 J/g after October 2013. The reason for this is not clear. The biodegradability was found to be in the range 50-350 mL CH4/g VS. It was shown that the sludge from Jurong pilot plant could be used to produce methane gas. In terms of g VS, the sludge biodegradability fluctuated around 200 mL CH4/g VS and was relatively stable after December 2013.

After the anaerobic biodegradability test, the residues were centrifuged and the centrate (<0.45 µm) was analyzed for SCOD, NH4+-N, TN, PO43-.
The SCOD was relatively constant in the range 150-300 mg/L. Ammonia-nitrogen was in the range 400-600 mg/L. The method for Total Nitrogen (TN) was changed in November 2013 and we used a Hach kit to improve our accuracy. The results showed that TN was in the range 400-800, therefore most of the residual nitrogen after the BMP tests was in the form of ammonia as expected. This is due to the breakdown of proteins to ammonia during the anaerobic digestion test.
Phosphates concentration was in the range 12-45 mg/L. We have ordered a new kit from Hach to analyze Total Phosphorus in these samples and the results showed that TP was very close to the PO43—P value, therefore most of the phosphorus was in the phosphate form. Unfortunately, the old samples showed a very low TP value which may be due to precipitation after long periods of storage in the cold room.

Unpublished report.