| Waste activated sludge(WAS) is a kind of hard biodegradable biosolids mainly produced by waste water treatment plant(WWTP) adopting biological process. It contains large amount of pathogen, parasites and heavy metal ions. Part of nitrogen and most phosphorus were enriched in WAS, if not disposed properly, the nitrogen and phosphorus could be washed back to water body. When treating WAS in conventional single phase mesophilic anaerobic digestion(AD) process, the process efficiency was generally poor due to the flock structure and cytoderm of WAS. Thermal hydrolysis(TH) was adopted as pretreatment method to destroy the cytoderm and release the readily biodegradable organic materials in order to increase the biodegradability of WAS. The process efficiency and economic efficiency of the combined TH and mesophilic AD process treating WAS were studied with conventional process as parallel.TH pretreatment was conducted under 175℃, 0.6~0.8 MPa and maintained 30 min. The results showed 48% increase in VS disintegrated rate and 7-8 times higher, 7800mgCOD/L in average in volatile fatty acids(VFAs) concentration. Biomethanation potential(BMP) of WAS were increased 43%. Reactor experiments showed that generally when combined with TH pretreatment, the TPAD process performed the highest efficiency, but the single phase mesophilic one was also of high efficiency and more practical for industrial application.Five progressively increasing volumetric organic loading rates(OLRs) ranging 1.74-4.27 gVS·L-1·d-1 were conducted in pilot-scale continuously stirred-tank reactors(CSTRs) with working volume of 190 L. The volumetric methane production rate and methane yield of combined TH process were 0.445-0.870 LCH4·L-1·d-1 and 0.255-0.197 LCH4·g-1VSin respectively, outperformed 80%-100% than they were 0.235-0.337 LCH4·L-1·d-1 and 0.153-0.110 LCH4·g-1VSin in conventional process. The specific methane production rate were 0.52 LCH4·g-1VSremoved in combined TH process and 0.37 LCH4·g-1VSremoved in conventional process respectively. The volatile solids(VS) reduction rate were 51.35%-38.18% in the former and 41%-29% in the latter respectively. The significant increment in biogas production was attributable to more organic materials which were more energetic after TH pretreated were degraded in the combined process.However, the combined process produced free ammonia(FA) ranging 89-382 mgN·L-1 in the digester, much higher than that(37-84 mgN·L-1) in the conventional process. Anaerobic microorganisms in both processes acclimated to high FA concentrations. The aceticlastic methanogenesis pathway was barely affected in the conventional process with FA increasing from 43 to 84 mgN·L-1, while the activity of aceticlastic methanogens was inhibited in the combined process with FA concentration higher than 264 mgN·L-1. The interaction of FA concentration, BMP utilization ratio, the inhibition of aceticlastic methanogenesis and VFAs accumulations indicated the combined process was mainly affected by nitrogen loading, which was different from organic loading limited conventional process. At high solid concentration, nitrogen loading rates should be considered as an limiting operational parameter for thermal hydrolysis anaerobic digestion process. This was different from the conventional one which was mainly limited by OLR.Depended on the feeding load data and methane yield of BMP and OLR experiments, the economic efficiency was studied in terms of total energy in biogas and steam generation costs. In China, when the required steam was generated by coal burning boiler, the combined process could benefit more, ranging 9.7-19.5￥/t. The inspirations for large WWTPs biogas plant were that the co-digestion with carbon rich materials such as food waste could alleviate FA inhibition, increase OLR and biogas production and hence reduce the cost of TH process operation. |
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