Evaluating the suitability of carbon source from fermentation of primary filter sludge for biological nutrient removal

Abstract: Traditional biological nutrient removal (BNR) processes in wastewater treatment require access to easily biodegradable carbon in order to achieve sufficient removal. However, a lot of the carbon in the influent wastewater is not directly available for BNR bacteria but need to undergo hydrolysis first. Several studies have investigated the possibility to produce an internal carbon source from the generated sludge at wastewater treatment plants (WWTP). The novel process combination of prefiltration and acidogenic fermentation of filtered primary sludge is investigated at a pilot plant at Källby WWTP in Lund, Sweden. The sludge is fermented in two reactors with different retention times, and contains a lot of volatile fatty acids, such as acetate and propionate. The suitability of the produced fermentate for usage in BNR processes was evaluated using batch activity tests. Nitrate uptake rate (NUR) tests were performed to determine denitrification rates and removal efficiencies with different substrates. The highest specific denitrification rates were obtained with the fermentates, in the range of 7-8.9 mg NO3--N eqv/g VSS·h, compared to 5.6 and 4.0 mg NO3--N eqv/g VSS·h observed in the tests with acetate and propionate respectively. A mixture of acetate and propionate gave rise to a similar rate as the fermentates (7.8 mg NO3--N eqv/g VSS·h), indicating that more complex carbon sources containing many different VFAs stimulates higher rates, as observed in other studies. Similar nitrate removal efficiencies were observed with all carbon sources, in the range of 83-95 % after 3.5 hours. In the test comparing influent wastewater with filtered wastewater, the highest denitrification rate was obtained with filtered wastewater (4.6 compared with 3.5 NO3--N eqv/g VSS·h), which could indicate a slight increase in readily available COD after filtration. However, the overall removal efficiency was higher in the test with influent wastewater (83% compared to 77%), reflecting the higher content of slowly biodegradable COD in the influent. Additionally, the removal efficiency of the filtered wastewater increased by 13% with the applied addition of fermentate (45 mg COD/L), also taking into account the reduced availability of the added carbon due to a small increase in nutrient loading. The total COD content was significantly lower in this case compared with influent wastewater, indicating a more efficient COD utilisation. The EBPR activity was tested in anaerobic P release tests. The observed P release rates were low. The tests were inconclusive to determine the suitability of the fermentate as a carbon source for EBPR. Overall, the fermentates showed good potential to be used as carbon sources for denitrification. More studies are needed to evaluate the applicability of the method in full-scale.