Integrating Waste to Energy at conventional WWTP’s David Leinster Bioenergy Association – Processing Food and Municipal Waste to Energy 19 th September, Hamilton, NZ
Integrating Waste to Energy at conventional WWTP’s
David Leinster
Bioenergy Association – Processing Food and Municipal Waste to Energy
19th September, Hamilton, NZ
Aquatec MaxconCompany Background
• Design and Construct Companyspecialising in Water and WastewaterTechnology
• In business continually for 47 years
• Over 60 anaerobic digestion projects
• Based in Ipswich, Australia
• Over 300 personnel, 70 engineers
• Offices in Ipswich, Sydney, Melbourne,Thailand, Indonesia
• In house fabrication, electrical, detaileddesign, installation teams
• Long term relationships with leadingglobal technology providers
Key Anaerobic Digestion Projects
• Coombabah – 100% WAS digester. Gold Coast, Australia
• Only type of its kind in Australia, very few similar plants globally. Achieves >30% VSR, operating since 2013.
• 20 day SRT/HRT
• Pre-thickening to minimise digester volume and feed heating requirements
• Concrete tank/floating steel roof design (50 year design life)
Key Anaerobic Digestion Projects
• Rewaste - Waste to Energy Facility. Melbourne, Australia
• 100T/d, dedicated facility processing commercial organic waste and powering adjacent WWTP with significant excess power fed to grid, operating since 2016
• Winner “Best international commercial plant” UK ADBA awards 2017
• Located next to an existing YVW treatment plant but does not process biosolidsfrom that facility
• Stainless steel panel tanks with double membrane roof design
Why consider Waste to Energy solutions at municipal WWTPs?
• Spare capacity - utilise existing digester capacity to increase gas production
• Generate revenue from waste producers to offset additional sludge processing costs
• Environmental benefits – reduce volumes of landfill, generate heat and power from waste streams reducing GHG emissions
• Develop synergies between traditional ‘waste’ and ‘water’ departments in municipal councils to solve problems
Anaerobic Degradation PathwayType of substrate affects pathway and treatment process
+ NH4 + PO4 + H2S
‘Typical’ Municipal WWTP approach to Co-digestion
Current approach
• Target small volumes of waste to utilise capacity in existing digesters and increase biogas production
• Target low nutrient, high FOG or high carbohydrate liquid waste streams due to nutrient loading concerns
• Typical waste sources include grease trap waste, soft drink waste, Liquid PDU waste
• Strong focus on risk management by not increasing the net nutrient content in digester centrate/filtrate stream returned to head of works
Limitations of current approach
• Sidestream treatment of high N & P stream is only viable at very large WWTPs
• Waste scope limited to small amounts of specific waste streams, often limited by FOG digester loading rate (foaming). Minimal revenue and biogas with this approach
• Many waste streams with high BMP, particularly food processing wastes (e.g. abattoir DAF sludges) are not considered suitable due to significant potential N & P load on WWTP when returned to head of plant as centrate/filtrate
Conventional WWTP with aerobic digestion
• Example configuration of TN <5mg/L, 4 stage WWTP with aerobic digestion for 20,000 EP.
Reconfigured WTE and capacity upgrade including primary treatment
and anaerobic digester
• Alternative configuration, implementing primary treatment and replacing aerobic with anaerobic digestion plus carbon substitution using organic waste
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Process reconfiguration – Primary Treatment and carbon substitution
• 1 - Implement primary treatment via rotating belt screen to reduce the slowly biodegradable BOD flowing to treatment plant (~30% lower total BOD) and treat this primary sludge stream anaerobically
• 2 - Replace COD lost by primary treatmentrequired for denitrification with a more
appropriate readily biodegradable liquidwaste stream (e.g. soft drink, brewery, foodprocessing (whey, molasses), glycerol,PDU
wastes) at no cost or a small net revenue
Process reconfiguration – High solids anaerobic digestion
• 3 - Implement pre-thickening for primary and WAS streams with ability totake in external waste streams for co-digestion. Significant benefits overrecuperative thickening, reduced digestate volume, lower heating cost
• 4 - Minimise centrate volumes and biogas production by operating at highsolids concentration. ~8-10% D.S. feed with upgraded mixing system tohandle high viscosity sludge
Example reconfiguration of 20,000 EP TN<5 plant to enable co-digestion
Parameter Conventional 20,000 EP Reconfigured 20,000 EP Percentage Change
Aeration volume required 3700 m3 1800 m3 Double hydraulic capacity
SRT 15.5d 15.5d Nil
Carbon dosing required? No ~180L sugar equivalent per day. ~2m3 of appropriate liquid waste equivalent
Potential revenue stream if appropriate waste stream can be sourced as carbon source
Sludge production 4.8m3/d at 13.9% D.S. 4.8m3/d at 13.6% Minimal difference
Nutrient load in returned centrate
Nil 20kg TKN/d
9kg TP/d
~8% of total nitrogen load on plant~20% of total phosphate load on plant
Power consumption (Aeration and digestion only)
1900 kWhr/d (35W/EP p.a.)
1150 kWhr/d (21W/EPp.a.)
40% reduction in power consumption
Potential heat and power Nil 1200 kWhr/d ~50kW CHP required –expensive $/kW installed cost. Consider co-digestion
Increasing biogas production with food waste co-digestion
Potential waste streams
• Supermarket organic waste (fruit/veg as well as out of spec/out of date food)
• Food processing waste (Chicken, beef abattoir and milk processing DAF sludges)
• Grease trap waste (limited amounts)
• Source separated commercial kitchen waste slurries (Pulpmaster or similar)
• Organic fraction of MSW generally uneconomic to recover without substantial landfill levies and Capex investments at transfer stations
• Without external carbon sources energy neutrality generally not possible with most existing secondary/tertiary WWTPs
• Compatible with solar, biogas can be buffered onsite and burned after solar production has peaked for the day
Opportunity – self sufficient WWTP
Managing high nutrient return stream – Digester Centrate
What happens if available feedstocks will significantly increase N&P in centrate? E.g. abattoir DAF sludge, high protein wastes, acid whey etc.
• Conventional approaches – sidestream treatment or return to head of works
• Alternative approaches – co-composting with green waste
???
Side-stream centrate treatment technologies
• Anammox
• Struvite recovery
• Ammonia stripping
• Chemical recovery options are high in Opex with generally limited revenue from recovered products such as MAP or ammonium sulphate
Conventional approaches to sidestreamdigester centrate nutrient recovery
Green Waste – An alternative method for processing high nutrient waste streams
Example City – 20,000 persons (Garden Organics only)
• Approximately 90kg green waste captured via green bins per person p.a.
• Currently mulched (at $5/m3 cost), to produce an unpasteurised low grade product with active weed seeds and termites for no revenue
• Mulched green waste C:N ratio around 100:1 with 25% moisture
• Green waste can be composted by adding water + nutrients
• Ideal range for composting C:N ratio 30:1 and 60% moisture
• Product is pasteurised value added and weed/termite free
• Approximately 20% load reduction on WWTP reducing Opex and extending asset life
Opportunity – utilise moisture and nutrients from digester centrate in the composting process
Example 20,000 person town
Centrate flow ~6.5m3/d
• 20,000 person @ 90kg green waste per person p.a. = 1,800 T/d• Mulched at bulk density 350 kg/m3 = 14m3/d at 25% moisture• Sufficient quantities of moisture and nutrient are available in centrate
to create perfect environment for windrow composting• Depending on location, significant moisture can be lost to evaporation
due to >60C temperatures in windrows• Value added, pasteurised product is produced which can be supplied
for road corridor rehabilitation etc.
Barriers to co-composting and risk mitigation techniques
• Perception – composting of digester centrate potentially viewed unfavourably. However unprocessed septic tank waste is currently co-composted on a large scale so there is significant precedence. AM also operate small resort WWTPs co-composting MBR sludge for gardens.
• Risk mitigation - Pasteurisation relatively simple via holding sludge at 70C for 1 hour using waste heat from CHP followed by aerobic composting for 2 barriers of pathogen reduction.
• Risk mitigation - Biosolids and composting guidelines – land application and risk mitigation techniques now well understood
• Risk mitigation - AM currently running trials at offsite composter to process municipal centrate to confirm economics. Transport major cost in processing large volumes so co-location of facilities into WWTP environmental buffer zones presents ideal opportunity
Managing Hydrogen Sulphide
• Organic waste can contain moderate levels of sulphide forming compounds which rapidly convert to hydrogen sulphide in anaerobic conditions
• Has been a barrier in past for biogas usage (e.g. recycled paper mills –biogas H2S up to 2.5%). ‘Typical’ H2S ~ 2,000 ppm.
• Traditional approach – dedicated scrubbers – biological, caustic, iron, activated carbon.
Managing Hydrogen Sulphide
• Alternative lower cost approach - Microaeration – currently installed in 2 industrial low rate digester plants with excellent results
• Utilises same mechanism as biological scrubbers in a much larger reactor at much lower biological load, 2,500 ppm -> 20ppm
• AM currently has a model for municipal digester market (floating roof digesters) currently in development with full scale trials starting Q4, 2017
• Measures and controls a range of parameters to optimise conditions for safety and performance
H2S + OH- HS- + H2O
HS- + 0.5O2 S + OH-
Thank You
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