Co-benefit biogas: Synergistic technologies enabling biogas use at smaller wastewater treatment plants. Stephan Heubeck, Jason Park and Rupert Craggs

Transcription

1 Auckland 14. August 2014 Co-benefit biogas: Synergistic technologies enabling biogas use at smaller wastewater treatment plants Stephan Heubeck, Jason Park and Rupert Craggs National Institute of Water & Atmospheric Research Ltd, New Zealand

2 Biogas and wastewater treatment Overlooked Problems A story of: Overlooked Opportunities

3 Previous assumptions From the text books: o Anaerobic digestion (AD) works well at ~ 35 o C and ~ 55 o C o Anaerobic activity rapidly decreases at below mesophilic temperatures o Unheated wastewater treatment and storage ponds are cold little biogas (methane) should be produced?

4 Source: H. Archer / BECA Previous assumptions Anaerobic digestion (AD) and biogas use: o Require heated and mixed digesters. o Is capital intensive and requires scale. o Therefore AD is only interesting for large set-ups o Benefits: Biosolids stabilisation / reduction Biogas energy BOD / COD removal?

5 Previous assumptions About anaerobic wastewater treatment ponds: + They reduce BOD/COD (50 75%) - They settle solids and produce lots of sludge - Methane emissions: GHG emissions Energy potential No idea, but can t be much??? - ANAEROBIC PONDS ARE ODOUR PROBLEMS!!!!!

6 Anaerobic pond field performance What do we really know about anaerobic pond performance treating municipal, industrial or agricultural wastewater in NZ?

7 Results dairy farm pond monitoring Southland: o Winter minimum temperature down to 8C. o No halt to methane production in winter o Reduced waste input during dry season

8 Results dairy farm pond monitoring Waikato: o Winter minimum temperature down to 10C. o No gap in methane production because no dry period o High correlation between waste input and methane output

9 Results dairy farm pond monitoring In a nut shell, treatment performance was: o Highly correlated to solids loading o Hardly influenced by temperature under the given conditions o Methane yields were similar to heated and mixed digesters o ~40% of COD load removed AND converted to CH 4 Southland Waikato No of cows cows Total pond load 33, ,066 kgvs/year Total pond load 53, ,468 kgcod/year Pond total annual CH 4 emi. 4,891 20,008 kgch 4 /year Pond total annual CH 4 emi. 7,301 29,863 m 3 CH 4 /year Methane productivity (VS) m 3 CH 4 /kgvs Methane productivity (COD) m 3 CH 4 /kgcod % COD conversion to CH 4 38% 44%

10 Baseline benefit Even discounting the biogas energy benefit completely (i.e. not recovering / using biogas) anaerobic digestion of wastewater has big advantages for WW treatment: Reducing aeration needs in aerated lagoons o Example, small town: 3,000m 3 /d 600 ppm COD o = 1,800 kg/d 40% reduction = 720 kg/d COD o = 720 kg/d O 1kWh/kgO 2 = 720 $0.15/kWh o = $ 108/d + 30 kw aerator capacity saved

11 Baseline benefit Reduced aerator demand alone can make a anaerobic pre-treatment pond stack up. Major Problem Odour Solution Cover and combust recovered gas

12 Using biogas Where is biogas use warranted: Local heat demand Always Electricity back-up needed Always Stand alone generation: o ~ 250m 3 CH 4 /day (= 50 kw 5,500 h/y) o Small town example: o 720 kg/d COD removed = 252 m 3 CH 4 /day o = 750 kwh/day biogas electricity o = equal to saved aerator electricity (720 kwh/day) The operational issues of using anaerobic pond biogas (condensate, corrosiveness, contaminants) are the same as with digester gas and solvable.

13 AD Conclusion Anaerobic wastewater treatment is almost always sensible: o Reducing aeration needs o Providing biogas energy even at modest scale (2x good) o Odours can be managed effectively The only real reason for not using anaerobic pond pre-treatment of wastewater is insufficient land being available

14 Nutrient removal Wastewater nutrient removal (N+P) is always difficult and expensive (energy intensive), but particularly at small scale Where sufficient land is available high rate algal ponds (HRAP) can be used to remove WW nutrients as algal biomass: o Low energy demand o Removes N + P o Co-benefit disinfection

15 Nutrient removal 5ha HRAP at Christchurch Wastewater treatment plant:

16 Nutrient removal 5ha HRAP at Christchurch Wastewater treatment plant: o Proved upscaling is possible o Nutrient removal to very low levels o Demonstrated simple algae harvesting technology successfully o Destroyed during earthquakes

17 Nutrient removal New 2ha system at Cambridge Wastewater treatment plant:

18 Nutrient removal HRAP systems: o Enable N+P removal at modest scale o Require little additional inputs (energy, chemicals) o Can provide cost effective nutrient removal + disinfection + BOD/COD polishing o 1 kg N removed, grows ~10 kg algal biomass, most of which is harvestable o What to do with harvested algal biomass? Anaerobic digestion to additional biogas!

19 Nutrient removal AD of algal biomass at Cambridge: o Accumulating pond digester digestate storage o Enabling nutrient reuse o Biogas co-used with WW treatment biogas o In theory total biogas / m 3 WW can be doubled

20 Summary Anaerobic digestion of WW can often make sense even without biogas use Simple pond covers address odour issues With simple anaerobic pond technology biogas utilisation can be facilitated at rather modest sized WW treatment plants Technology works stable in NZ climate and is applicable to municipal, industrial and agricultural wastewaters HRAP an interesting option for nutrient removal at modest scale that can provide additional biogas