Thermal hydrolysis and thermal alkaline pretreatment of waste activated sludge: comparison of effects on biogas yield, dewaterability, sludge liquor and cost-benefit analysis

Aim of this dissertation was to investigate effect of process conditions (T, pH) of thermal alkaline pretreatment (TAP) and thermal hydrolysis (TH) of waste activated sludge (WAS) on biogas yield increase, dewatering potential of digestate, return load increase and refractory COD using same sludge. Moreover, it was intended to compare economic viability of TAP and TH for an individual wastewater treatment plant (WWTP) with fully comparable conditions. Higher initial anaerobic biodegradability of WAS leads to lower increase in biogas after pretreatment. Based on WAS initial biodegradability, 22-97% increase in biogas can be expected. Treatment temperatures below 100°C can be as effective as temperatures above 100°C in terms of biogas increase. Alkali dosage and resulting initial pH play a significant role on biogas increase. It is recommended to keep NaOH dosage in range of 40-60 mg per g total solids (TS) in sludge. Treatment time of 1.5-5.0 h suffices, and longer treatment times show no extra positive impact on biogas yield. Effects of TAP on dewatering potential and polymer consumption are still ambiguous and more systematic investigations using standard methods or full-scale dewatering equipment are needed. Low temperature TAP of WAS in pilot-scale showed a sinusoidal trend for biogas production throughout the year. Biogas yield increased maximum +42% in summer and minimum +3% in winter resulting an average yearly increase of +20% (by digestion of primary sludge+WAS). Ammonium and orthophosphate in sludge dewatering liquor increased by +34.6%, and +17.0%, respectively. Dewatering potential showed no improvement in terms of TS of centrifuged digested sludge. Aerobic biodegradability test of sludge dewatering liquor showed 30.3% increase in refractory COD. Modeling via data from six WWTPs in Berlin showed that TAP leads to 0.8-1.1 mg per L increase in effluent COD. TH of WAS in temperature range of 130-170°C in lab-scale showed an increase of 17-27% in biogas. Dewatering potential tests via lab-scale centrifugation demonstrated 12-30% relative increase in TS of dewatered cake (+4% in absolute TS). Test of aerobic biodegradability indicated that refractory COD in sludge dewatering liquors can increase by 50-200%. It was shown that with reducing temperature from 170 to 160°C, refractory COD is halved, while 19% increase in biogas and 20% relative increase in TS of dewatered cake is still achievable. Modeling via data of six WWTPs in Berlin showed that effluent COD increase of 2-15 mg per L after TH is expected. Mass and energy balances of an individual WWTP showed TH results in increase of digestion capacity, while TAP has no effect on it. Furthermore, TH results in a slightly higher total biogas production than TAP. On the other hand, increase of refractory COD and consequently increase of effluent COD of WWTP is higher by TH and low to negligible by TAP. Moreover, TH results in higher reduction of disposal costs than TAP due to improvement of dewatering potential. On the contrary, polymer costs increase by TH. TAP and TH both enhance electricity and heat balance of WWTP. Capital costs of TH are 4-5 times higher than TAP. Both TAP and TH yield high revenues due to reduction of total operating costs. Specially for lower biogas increase, TAP yields higher revenue than TH. With increase of biogas potential due to pretreatment, TH slightly surpasses TAP in yielding revenue. Finally, TAP and TH are both profitable and other key factors such as capital costs, refractory COD formation, digestion capacity increase or simplicity of process can be considered by decision making.

Author dc:creator

• Toutian, Vahid

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-18220

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/19423