Carbon utilisation for extended nitrogen removal and resource savings

Författare

Tobias Hey

Summary, in English

A full-scale in-line primary sludge hydrolysis experiment was conducted in one out of four primary settlers at the Klagshamn Wastewater Treatment Plant (WWTP) to test if the wastewater quality can be improved in terms of providing easily accessible carbon for possible pre-denitrification and the reduction of external carbon sources. The amount of easily accessible carbon produced, in the form of the volatile fatty acid (VFA), alkalinity and ammonium concentrations, was measured throughout the entire full-scale experiment at the outlet of the hydrolysis tank and that of the ordinary primary settler, which served as a reference line. VFA concentrations were measured in wastewater and hydrolysate samples using three analytical methods: the 5 and 8 pH point titration methods and gas chromatography. A calibrated model was established to fit data regarding the Klagshamn WWTP’s annual activated sludge operation of its secondary settler and wastewater composition. For modelling purposes and due to the small amount of data available, a linear regression method was established and used to complete the annual data set of the wastewater entering the Klagshamn WWTP. The full-scale data were incorporated into the calibrated model to simulate different scenarios of the activated sludge process with the purpose of saving energy (electricity) and resources (ethanol). Furthermore, an environmental (CO2-emissions) and economic evaluation was performed based on the data gathered from the full-scale experiment.

A VFA concentration of 43 mgCODVFA∙l-1 with no release of ammonium was achieved in the full-scale hydrolysis experiment; this amount was shown, by simulation, to substitute for 50% of the concentration of ethanol currently used. The amount of ethanol saved represents an equivalent electricity saving of 19 MWh for ethanol production, and the operation of fewer nitrification zones, while still maintaining full nitrification over two summer months, could ensure an additional electricity saving of 177 MWh. The evaluation and comparison of the results obtained using the three techniques showed that the 5 pH point titrimetric method was adequate and sufficiently accurate in this context to monitor VFA concentrations below 100 mg∙l-1 at an alkalinity of 300 mgCaCO3∙l-1. The method can be easily implemented in the routine laboratory of the WWTP, and the measured VFA concentrations are equivalent to those obtained by gas chromatography.

For the Klagshamn WWTP, the modelling results and further evaluations showed that in-line primary sludge hydrolysis can decrease the dependence on external carbon utilization and can thereby reduce chemical costs and carbon dioxide emissions.

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