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

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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

• Please come and say hello afterwards!