Abstract
Anaerobic co-digestion of organic rich wastes and wastewater sludge has become an attractive economic possibility for water utilities as it enhances biogas production. The suitability of the organic rich waste depends on its biodegradability as well as on its synergetic effect on the anaerobic digestion process. The feasibility of sewage sludge (SS) treatment via co-digestion was studied in a semi-continuous mode at mesophilic conditions (36 ± 1 °C), with a hydraulic retention time of 17 days and an average organic loading rate of 0.94 ± 0.05 gVS Lreactor day−1, using the liquid fraction of pre-treated exhausted coffee biowaste (LECB) as a co-substrate. An anaerobic co-digestion trial (T1) was performed using as feeding mixture 80% SS and 20% LECB (v:v) and compared against a reference scenario of mono-digestion of SS (T0). The stability along assays was ensured by monitoring the digestate characteristics (pH, electrical conductivity, total alkalinity and ammonia content) and the specific energy-loading rate (SELR). Along the operation time of T1, methane yield and VS removal were significantly higher in comparison to mono-digestion of SS. Results showed that the addition of the co-substrate had a positive effect on specific methane production (3 times higher) and methane content (12% higher), indicating this is a feasible strategy towards self-sufficient wastewater treatment plants.
Highlights
The circular economy leads to a new vision of wastewater treatment plants (WWTP), considered as resource recovery factories [1, 2]
This study supports the implementation of anaerobic codigestion of sewage sludge (SS) and liquid fraction of pre-treated exhausted coffee biowaste (LECB) as a strategy towards an enhanced bioconversion process
It was possible to conclude that anaerobic co-digestion (AcoD) process using a feeding mixture of 80% SS and 20% LECB (T1), keeping the same hydraulic retention time and similar organic loading rate, was optimised compared to the reference scenario (T0) using SS as mono-substrate
Summary
The circular economy leads to a new vision of wastewater treatment plants (WWTP), considered as resource recovery factories [1, 2]. WWTP generates large amounts of sewage sludge (SS) that can be used to recover bioenergy through anaerobic digestion (AD). The poor biodegradability of SS leads to low methane yields and it is crucial in defining appropriate waste management scenarios to enhance biomethanisation. This can be achieved either by pre-treating the SS [3] or selecting different types of biowaste with high methanogenic potential to be codigested with SS [4,5,6,7,8,9]. The optimisation of energy recovery through AcoD enhances WWTP environmental performance and lowers its carbon footprint [11]. Implementation of AcoD in WWTP along with other measures, for example optimisation
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