Methods of anaerobic digestion of biomass to produce biogas
Improved methods for anaerobic digestion of organic matter to produce biogas. Among the improvements given are including ferric iron in a hydrolysis reactor to increase the rate and efficiency of anaerobic hydrolysis to provide substrates for methanogenesis. A solids separation step is added after hydrolysis and before methanogenesis to improve the efficiency of the methanogenesis step. Other improvements involve using separate tanks for the hydrolysis and methanogenesis stages and using two (or more) methanogenesis tanks in sequence, and switching the order of the two (or more) methanogenesis tanks periodically.
1. A method of producing biogas comprising:
(a)(i) grinding an organic substrate comprising solids to produce smaller solids particles;
(a)(ii) transferring the substrate with smaller solids particles to a hydrolysis tank;
(a)(iii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank comprising Fe 3+ particles;
(a)(iv) transferring effluent from the hydrolysis tank to a solids separator
(a)(v) separating solids from the hydrolysis effluent to produce a solids fraction and a screened hydrolysis effluent;
(b)(i) transferring the screened hydrolysis effluent for a period of time 1 to a methanogenesis tank 1 comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank 1 liquid effluent;
(b)(ii) transferring the methanogenesis tank 1 liquid effluent to a methanogenesis tank 2 comprising fixed film methanogens or granular methanogens during period of time 1 and producing biogas and methanogenesis tank 2 liquid effluent in methanogenesis tank 2 ;
(b)(iii) discharging methanogenesis tank 2 liquid effluent during period of time 1 ;
(c) and then switching the order of methanogenesis tanks 1 and 2 and
(c)(i) transferring the screened hydrolysis effluent for a period of time 2 to said methanogenesis tank 2 and producing biogas and methanogenesis tank 2 liquid effluent;
(c)(ii) transferring the methanogenesis tank 2 liquid effluent to said methanogenesis tank 1 during period of time 2 and producing biogas and methanogenesis tank 1 liquid effluent in methanogenesis tank 1 ;
(c)(iii) discharging methanogenesis tank 1 liquid effluent during period of time 2 .
2. The method of claim 1 further comprising:
(d) switching the order of methanogenesis tanks 1 and 2 and repeating steps (b)(i), (b)(ii), and (b)(iii).
3. The method of claim 1 wherein the step of separating solids uses a solids separator that is a screw type solids separator.
4. The method of claim 1 wherein the period of time 1 and the period of time 2 are each between 6 and 24 hours inclusive.
5. The method of claim 1 wherein the hydrolysis tank has a hydraulic retention time of 96 hours or less.
6. The method of claim 1 wherein the methanogenesis tank 1 and the methanogenesis tank 2 each have a hydraulic retention time of 48 hours or less.
7. The method of claim 2 further comprising monitoring oxidization-reduction potential (ORP) in methanagenesis tank 1 and/or methanogenesis tank 2 , wherein the methanogenesis tank receiving the hydrolysis effluent is the lead methanogenesis tank, and the other methanogenesis tank, receiving methanogenesis tank liquid effluent from the lead methanogenesis tank, is the lag methanogenesis tank, wherein the switching step (c) occurs when the ORP in the lead methanogenesis tank rises to a preset value or the ORP in the lag methanogenesis tank falls to a preset value.
8. The method of claim 7 wherein the switching step (c) occurs when the ORP in the lead methanogenesis tank rises to a preset value between about −400 and −300 mV or the ORP in the lag methanogenesis tank falls to a preset value between about −450 and −550 mV.
9. The method of claim 2 wherein time period 1 and time period 2 are preset time periods.
10. The method of claim 1 wherein the sum of the hydraulic retention times for the hydrolysis tank and methanogenesis tank 1 and methanogenesis tank 2 is a total hydraulic retention time, and wherein the total hydraulic retention time is 8 days or less.
11. The method of claim 1 further comprising:
(d) passing the biogas produced in methanogenesis tanks 1 and 2 through a foam trap to trap foam and separate it from the biogas; and
(e) collecting the biogas downstream of the foam trap.