Analysing impacts of thermal hydrolysis on biogas production at Grevesmühlen WWTP
Project activities related to Grevesmühlen WWTP have been carried out by two project partners: with the investment hosted by the plant (Joint body Grevesmühlen Water Supply and Wastewater Disposal, Germany), a decision making tool for optimized process operation and mass flow management has been prepared by the aqua & waste International GmbH company. To calibrate the developed tool, Grevesmühlen WWTP launched investigation of the process under varying conditions, focusing particularly on thermal hydrolysis.
The aim of thermal hydrolysis is to destroy the membrane of bacterias in the excess sludge with the help of high pressure and temperature. Through this process, water and nutrients within the bacterias are set free. One of possible benefits of performing thermal hydrolysis can be reduced amount of sludge mass in the digestion. Reduction of sludge mass leads to a higher detention time, which in turn can result in a higher biogas production and a better dewatering capacity of the digested sludge. However, there might be also disadvantages, as it is not investigated, how many nutrients are set free from the sludge as a function of pressure and temperature.
On a laboratory scale, an anaerobic digestion was simulated with sludge from both a laboratory scale and an industrial scale thermal hydrolysis unit. Each sludge-batch was hydrolyzed with a different combination of pressure and temperature. Several parameters were measured each day, for example, sludge temperature, COD, nitrogen, phosphorus, dry matter and organic dry matter before and after the digestion. Moreover, biogas parameters like the amount of produced biogas, CH4, CO2, H2S and O2 were tested. Each sludge batch was digested up to 30 days.
In the course of the observation on the laboratory scale, the amount of additional loads set free through hydrolysis was higher with decreasing temperatures of the sludge. At the same time, on the industrial scale it was observed the other way around, which was the expected result. Another unexpected outcome was the lower amount of the produced biogas from hydrolyzed sludge than from raw sludge. The reason for this can be that the hydrolyzed sludge has more loads, while the second step of the anaerobic digestion – the acidification – produces too many acids, which inhibits the whole anaerobic process.
The sludge was examined for organic dry matter and dewatering capabilities. On the laboratory scale, the results met the expectations proving that hydrolyzed sludge has a better dewaterability than raw sludge.