1. University of Southampton Science Park
Flexibuster Anaerobic Digester supplied by SEaB Energy
Introduction
This facility operates as a treatment hub serving several organisations. It is a example of a technology that would potentially be suitable for the
on-site treatment of organic waste arising from a large hotel or hospital. Southampton University Science Park houses more than 50 companies (900 regular staff members) in 45 acres of landscaped grounds and adjoins the village of Chilworth on the outskirts of Southampton. The Anaerobic Digestion unit is SEaB Energy’s main demonstration facility and accepts organic wastes from a number of local organisations. Businesses at the science park are provided with food waste caddies which are left outside the unit when ready for disposal. A similar arrangement is in place for residents of Chilworth who can use the facility free of charge as long as they deliver waste to the unit. Food waste is also provided by the nearby Chilworth Manor Hotel and The Chilworth Arms public house.
Nature and quantity of waste treated on site
The Flexibuster treats food waste including cooked and uncooked meat/fish plus grass cuttings from the estate grounds.
The majority of the waste collected was previously sent to landfill.
Approximately 2.5 – 3.5 tonnes of organic waste is treated per week.
Key features
Size: 12 x 2.44 x 2.8 m (pasteurisation tanks and digester), 6 x 2.44 x 2.8 m (biogas storage) and Small CHP unit
Spatial requirements: Area to contain above with 1m exclusion zone on all sides.
Capacity: 350 – 500 kg food waste per day
Energy/ Water requirements: 5-10% of output electricity is needed for parasitic load, 60-70% of output heat and ~22m3 grey water/year is used in the process
Housing: On hard standing (non-porous) with fencing to form 1m exclusion zone
Maintenance: Annual servicing / general maintenance of AD unit and CHP unit
Output: Fertiliser, Mulch and biogas generated in 15 days. Biogas burnt to produce heat and power.
“We are delighted to work in partnership with SEaB Energy and to reaffirm our commitment to providing the benefit of innovative green technology to tenants”
Peter Burkett, Chief Executive, Southampton Science Park
SEaB ‘Flexibuster’ MB400AD in Container

2. How food is separated for treatment
University of Southampton Science Park: Flexibuster AD
Lessons Learnt
Filtration has been installed for the hopper to reduce the smell from input waste; pasteurisation tanks were initially larger than required and have been reduced to match feedstock input; the smallest suitable Combined Heat and Power (CHP) unit was around twice the size needed for this scale of facility, so it only operates ~50% of the time and gas storage is required when CHP not running, heat and electricity output are therefore not continuous.
How food is separated for treatment
Food waste is delivered in food caddies from local residents and science park businesses. Larger bins are provided by the local hotel and pub. Grass cuttings are collected by grounds maintenance staff and delivered to the unit.
The waste is inputted into the facility daily. Food and grass waste is added together, however there is no need to mix or ratio the different wastes.
10 minutes is required for the daily feeding process.
Use of outputs from treatment
The digestate is used on the science park grounds, with potential to use as fertiliser at a nearby turf growers.
Biogas is produced (reported to be between 70-78% methane consistently) and burnt using a 8kW CHP unit to produce 70MWh/annum electricty and 140 MWh/annum heat.
Electricity from CHP unit is exported to office developments at the science park. 5 – 10% of electricity is used to operate the unit.
Excess heat from CHP unit is intended to be exported to neighbouring greenhouse (c. 10 meters away) and 60% utilised by the process.
How the on-site treatment system is managed
Approximately 728 hours staff time per year is required to run the digester (~2 hours a day) on-site. Off site support and monitoring is provided by SEaB.
Grounds maintenance staff who collect grass cuttings also operate the facility.
The waste is fed into a hopper, passed through 2 pasteurisation tanks and a digester tank before the resulting biogas is stored and utilised in a Combined Heat and Power (CHP) Engine every other day.
Waste is digested in 15 days and turned into digestate and methane.
External assistance is available from SEaB if required and initial training is provided for the operators.
No significant health and safety concerns have arisen.
Costs
Installation and start-up costs for the Flexibuster start at £115,000. A leasing arrangement is available, which includes maintenance and support, for £6,500 per annum for this scale of facility.
A significant cost saving is realised from displaced energy costs.
Labour costs for the operation of the unit are estimated to be roughly £20/day (2 hours in total for feeding, cleaning and general maintenance).
Savings are currently estimated at £150/tonne based on displacement of disposal costs.
The production of digestate/fertiliser is seen as a perk rather than a replaced product purchase.
Payback period for the digesters is reported to be between 2 and 6 years dependent on waste compositions, existing energy bills, existing waste disposal costs etc.

3. www.wrap.org.uk/on_site_treatment Waste & Resources Action Programme
This case study is part of a series of case studies focussing on the on-site treatment of organic waste. Other case studies in this series are:
Eriska Hotel;
Housing 21;
Millets Farm Centre;
Dartington Primary school;
University of Bradford; and
Her Majesty’s Prison Service
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