Treatment of Process Water of hydrothermal carbonised Sewage Sludge before discharging in a Waste Water Treatment Plant

Abstract

Sewage sludge is currently more often thermally treated due to problematic ingredients. So far the necessary drying was done by thermal methods with high inputs of energy. Hydrothermal carbonisation (HTC) is presented as CO2 saving and cost efficient alternative to dry sewage sludge on industrial scale. Further processing of HTC process water before discharging it in a waste water treatment plant is a critical point in the carbonisation of sewage sludge concerning costs and environmental compatibility. The potential for biological decomposition was investigated in HTC process water and permeate, which was gained by membrane filtration of HTC process water. A medium to high removal efficiency was shown for organic matter in HTC process water and permeate by aerobic and anaerobic processes in batch experiments. In a continuous aerobic laboratory reactor the HTC process water and permeate showed removal efficiencies of 70–75 % for chemical oxygen demand (COD) at a specific load of 5–10 kg COD/(m3 reactor volume ∙ d). HTC permeate produced by optimised membrane filtration (cut-off 96 nm) showed improved removal efficiencies in first experiments. As HTC process water and permeate contain toxic or inhibiting components regarding to nitrification the co-treatment of process water or permeate in a wastewater treatment plant with nitrification needs diluting the process water or permeate at least 100 fold in order to prevent inhibition of the nitrification process.