Food producer doubles its reuse capability
Water reuse is an increasingly attractive process for food manufacturers across the UK and worldwide. Geraint Catley and Steve Goodwin of Aquabio revisit an expanding site in Evesham to increase capacity
With the stigma of wastewater reuse in food factories much reduced, the driving issues for the sector are now linked to the economics of recycling water.
Charges for incoming water are rising, as are effluent disposal costs. This is in addition to limitations on borehole/surface water extraction or final discharge constraints.
Recycling is therefore an increasingly attractive option and with recent developments in treatment technology, the payback on capital investment has reduced significantly, in many cases to below two years.
These motivating factors are heightened by business growth and/or factory expansion and pressure from customers to reduce carbon and water footprint.
Companies with existing wastewater pre-treatment infrastructure may find investment in potable recycling option more attractive. The UK Government is offering an Enhanced Capital Allowance (ECA) tax incentive scheme for water reuse investments where 100% of capital expenditure is fully allowable in the year following start-up of an ‘eligible’ efficient membrane-based plant reusing >40% of the factory wastewater.
Aquabio has specialised in industrial water reuse since 1998 and uses its own crossflow membrane bioreactors (MBRs) as the core process in its wastewater recycling plants. By developing three types of crossflow based MBR technology, the company can offer its clients the most cost effective alternative for their particular circumstances.
All Aquabio MBR plants have ‘true’ ultrafiltration (UF) membranes mounted outside of the bioreactor in dry conditions which means hygienic operation, ease of maintenance and no parts immersed in the biomass. The three types of system are:
AMBR – which utilises high crossflow to achieve high flux rates. The bioreactor can be operated at high biomass mixed liquor suspended solids (MLSS) concentrations. The technology is aimed at lower flow and higher strength wastewaters. Plants are extremely compact, with very low membrane area and low membrane replacement costs.
The membrane banks are operated automatically based on level in the bioreactor and hence are utilised, as required, to match the inlet flow. When not required, individual membrane banks are automatically shutdown, auto-flushed to remove biomass and await re-start thus saving energy when the plant is not running at full design, or when membrane flux is higher than design. AMBR plants are modular, can be containerised and are relatively low in capital investment cost.
AMBR LE – which utilises intermittent backflushing for the control of membrane fouling and hence allows a reduction in crossflow velocity and a significant saving in energy. Medium to high MLSS concentrations are accommodated and AMBR LE is aimed at medium to high flow and low to high strength wastewater. With variable speed recirculation pumps the system allows for a ‘managed’ flux rate which is used to optimise energy consumption relative to the plant load.
Plants remain very compact, with low membrane area and moderate membrane replacement costs. Plants also operate on a direct link to level in the bioreactor, automatically adjusting membrane permeate production (and energy use) to suit the inlet flow conditions. AMBR LE is particularly suited to applications with inconsistent or variable wastewater flows, high peak or seasonal loads, or when electricity costs are moderate to high.
BIOVERT – Incorporates vertically mounted modules and uses air injection to further reduce energy consumption. This process allows for medium MLSS concentrations and is aimed at higher flows and low to medium strength (i.e. mainly larger municipal applications). Plants are compact, with vertically mounted membrane modules and have more installed membrane area.
Plants operate on a more constant level in the bioreactor, with controlled air injection at either the top, or the bottom, of the membrane tubes. Air scour and turbulence reduces surface fouling and energy use is very low.
BIOVERT plants are particularly suited to applications with large consistent wastewater flows and when electricity costs are high.
Operations input is low for factory personnel thanks to Aquabio’s proprietary control software and remote real time monitoring is a standard feature using internet links. All plants have automatic operation including, flushing, backflushing, cleaning, start up and shutdown on demand.
Case Study: Kanes Foods, Evesham
In 2001 Kanes Foods in Evesham installed an Aquabio ‘first generation’ AMBR plant combined with reverse osmosis (RO) and UV disinfection for wastewater reuse at potable water standards. On the initiative of the company’s owner, John Randall, and extensive trial work over the preceding 18 months, this became the first food plant in Europe to recycle treated wastewater on a large scale producing high quality, potable water.
Due to factory growth and continuing limitations on water supply and discharge options, Kanes have recently installed a second AMBR LE plant on the same site to increase the treatment capacity and recycle volume.
The 2001 first generation AMBR process treatment scheme comprises upstream screening, flow balancing, dissolved air flotation (DAF) treatment for fine vegetable solids removal, the AMBR itself and downstream treatment by RO, followed by UV disinfection. This process yields potable quality water suitable for blending with mains water for use within the factory.
The process comprises two 250m3 bioreactors with four banks of crossflow membrane modules. The maximum MLSS concentration employed has been as high as 22g/l, but the bioreactor is generally operated at around 10g/l.
Sludge production is calculated as being 0.14kg DS/kg COD removed at a sludge age of over 100 days. Each membrane bank comprises four UF membranes, which operate at an average flux of 153l permeate production/m2 membrane area (LMH) normalised to 25°C. The permeate water has average TSS, BOD and COD concentrations of only 4, 7 and 16mg/l respectively.
The UF permeate then passes to a two-stage RO plant which achieves an overall recovery of 75%. The reject stream is discharged to sewer and the permeate, which typically has a conductivity of 40-100 µS/cm, is passed to the UV disinfection unit and on to the client’s water supply tank.
The original plant has now been in continuous operation for nine years and has performed consistently in terms of biological treatment, membrane performance and final reuse water quality. Mostly membrane performance has been better than design allowing one bank to be maintained as a standby and so offering more process flexibility and lower energy use. Occasional reductions in membrane flux have been linked to poor biomass health which has been rectified by closer management of the process.
Due to production expansion and the demand on incoming water supply and discharge consent, a second wastewater treatment plant was constructed by Aquabio and commissioned in February. This plant provides an additional 1.435Ml/d of biological/MBR treatment capacity and an additional 0.6Ml/d of reuse water.
The plant replicates the successful process scheme of the existing plant, but utilises Aquabio’s AMBR LE technology to provide significant energy savings compared to the original plant. New common inlet screening and flow balancing facilities are provided, which allows the original balance tank to be used to balance outgoing flows to sewer.
Again, DAF treatment is employed to remove fine vegetable solids prior to the AMBR LE system. A single bioreactor of 765m3, allowing for MLSS concentrations up to 12g/l, is aerated by a blower assisted slot type aeration system, with high alpha factor characteristics.
The AMBR LE UF membrane system comprises four banks of 4m long membranes. Low energy performance is achieved by the use of backflushing to control membrane fouling thus allowing much reduced cross flow velocities and hence significantly lower energy use.
The combination of backflushing frequency and variable cross flow velocity is optimised to give the lowest energy use for the required throughput. Significant process flexibility is offered by the inclusion of variable speed recirculation pumps and optional permeate pumping to control the membrane trans-membrane pressure (TMP) and hence the flux performance.
Stable performance is being achieved at an energy use of 0.3-0.5kWh/m3 of treated permeate for the biomass separation, a significant improvement on the original plant. By the end of March 2010 both Aquabio plants had recycled 1.68B litres of potable quality water back into the factory since their start up. Additional quantities of AMBR permeate water have also been reused in non potable duties.