New wastewater treatment concepts towards energy saving and resource recovery

At present, conventional activated sludge (CAS) systems are widely applied to treat municipal wastewater. The main advantages of CAS systems are that they are robust and generally produce an effluent quality that meets the discharge guidelines. However, CAS systems cannot be considered sustainable because they consume large amounts of energy (mainly for aeration and sludge treatment), have a high CO2 emission and do no recover a potential resource of water, energy and nutrients nitrogen (N) and phosphorus (P). Therefore, in this thesis new municipal wastewater treatment concepts that combine treatment with recovery of valuable resources and can save considerable amounts of energy were investigated by modelling and experiments.

Quantitative numerical results showed that the feasibility of two novel wastewater treatment configurations, including combined bioflocculation and anaerobic digestion but with different nutrient removal technologies, i.e. (cold) partial nitritation/Anammox or microalgae treatment, is location dependent. Using Dutch municipal wastewater and climate conditions, the configuration with cold partial nitritation/Anammox is the most promising wastewater treatment concept, because it can: 1) treat wastewater year round; 2) produce an effluent at a quality that meets the discharge guidelines; 3) reduce CO2 emission by 35% compared to the CAS system; 4) achieve a net energy yield up to 0.24 kWh per m3 of wastewater compared to a negative net energy yield of -0.08 kWh per m3 of wastewater for the CAS system; and 5) recover 80% of the sewage P. Additionally, the feasibility of the two configurations was investigated for 16 locations around the globe. The results quantitatively support the pre-assumption that the configuration with (cold) partial nitritation/Anammox is applicable in tropical regions and some locations in temperate regions. The configuration with microalgae treatment is only applicable the whole year round in tropical regions that are close to the equator line. The results also showed that the configuration employing microalgae treatment has an advantage over the configuration employing partial nitritation/Anammox with respect to consumption of aeration energy and recovery of nutrients, but not with respect to area requirements. For a tropical climate country like Thailand, the net energy yield of both configurations is at least a factor 10 higher than the CAS system, while CO2 emission is at least 22% lower.

In CAS systems energy recovery from wastewater is accomplished by anaerobic digestion of the organic solids in primary and secondary sludge into methane. However, volatile fatty acids (VFA), which are intermediate digestion products, may be preferred over methane, because VFA can be used as starting compounds for a wide range of higher value products. In this thesis the experimental results showed that a combined process with bioflocculation, using a high-loaded membrane bioreactor (HL-MBR) to concentrate sewage organic matter, and anaerobic fermentation, using a sequencing batch reactor to produce VFA is technologically feasible. An HL-MBR operated at a hydraulic retention time (HRT) of 1 hour and a sludge retention time (SRT) of 1 day resulted in very good performance, because as high as 75.5% of the sewage COD (chemical oxygen demand) was diverted to the concentrate and only 7.5% was mineralized into CO2. It was also found that 90% of the sewage NH4-N and PO4-P were conserved in the HL-MBR permeate, which can be reused as irrigation water as it is free from solids and pathogens.

During anaerobic fermentation of the HL-MBR concentrate at an SRT of 5 days, 35°C and without pH control, methane production was inhibited, but incomplete solids degradation mainly limited the VFA production as only 15% of the sewage COD was converted to VFA. Thus, the VFA yield needed to be increased. It was hypothesized that high pH (pH 8–10) fermentation combined with a long SRT, allowing for sufficient solubilization of solids and colloidal COD, can improve the VFA yield. In the current study, it was found that application of a pH shock of 9 in the first 3.5 hours of a sequencing batch cycle followed by a pH uncontrolled phase for 7 days gave the highest VFA yield of 440 mg VFA-COD/g VSS and this was equivalent to 26% of the sewage COD. This yield was much higher than at fermentation without pH control or at a constant pH between 8 and 10. The high yield in the pH 9 shock fermentation could be explained by: 1) a reduction of methanogenic activity; or 2) a high degree of solids degradation; or 3) an enhanced protein hydrolysis and fermentation. This study also demonstrated that the VFA yield can still be further optimized by fine-tuning pH levels and longer operation, possibly with fermentative microorganisms adapted to a high pH that are commonly found in nature. This would further increase VFA yield to 33% of the sewage COD.