Membranes boost water re-use
Over half of total factory water at Kane Foods is now re-used following treatment by an advanced membrane bioreactor, write Steve Goodwin and Geraint Catley of Aquabio
With increasing pressure on factory water usage due to expanding production,
one UK food processing company is now re-using up to 650m3/day of its trade
wastewater stream (approximately 55% of the total factory volume) to reduce
water usage and discharge costs.
The use of mains water and eventual discharge of the trade wastewater to the
public sewer meant increasing pressure on incoming water use and discharge costs
as well as hydraulic limitations to the sewer. This led to Kanes considering
the treatment of the trade wastewater to very high standards to enable re-use
within the production areas. Due to space limitations, a compact, small-footprint
plant was required, based on a robust process and offering reliable operation
without the need for specialist knowledge.
A pilot-plant trial was conducted by Aquabio, the Worcester-based water treatment
company, to evaluate the feasibility and process sequences required to achieve
the necessary treatment and to collect operational data. The trials were conducted
on part of the factory discharge for a period of six months between January
and June 1999. The results enabled a full-scale engineering design to be developed
leading to Aquabio constructing and installing a turnkey wastewater treatment
and re-use facility, capable of receiving 1,200m3/day of raw wastewater and
providing 650m3/day of fully treated potable-quality water for re-use within
At the heart of the wastewater treatment plant is Aquabio’s advanced membrane
bioreactor (AMBR) process. This is an aerobic biological treatment system comprising
an intensive activated-sludge process with the biomass separation stage carried
out by ultra-filtration (UF) membranes.
The UF membranes replace the settlement stage in conventional activated-sludge
systems and effectively revolutionise the process. The separation of biomass
from treated water using membranes not only provides filtered quality final
effluent, offering possibilities of re-use, but also allows very high biomass
mixed liquor suspended solids (MLSS) concentrations to be developed in the bioreactor
without the detrimental effects usually associated with traditional settlement
techniques. In turn the high biomass concentrations (15-20kg/m³) allow
the bioreactor volume to be reduced to as low as 20% of the size for conventional
treatment. The high MLSS concentration allows the system to be run at very high
sludge ages and facilitates concurrent sludge digestion within the bioreactors,
hence giving low sludge yields, typically 0.25kg DS/kgCOD removed. With simple
storage and decant the waste sludge can be disposed of at consolidated concentrations
of around 3% dry solids content.
At Kanes Foods the AMBR process takes the form of two 250m³ bioreactors
and four banks of UF cross-flow membranes offering a total of 312m²
of membrane surface area. The two bioreactors allow the process flexibility
to operate the system in series or parallel. Currently series operation is utilised
successfully to provide roughing treatment in the first tank followed by polishing
in the second. The bioreactors are mixed and aerated by two Jetox aeration systems
providing a total of 1,890kgO2/day. The aeration system is automatically controlled
by the PLC to achieve the necessary dissolved oxygen (DO) residual.
The external cross-flow membranes offer much greater permeate flux rates than
submerged gravity type membranes, typically 80-120 l/m2/hr, although flux rates
at Kanes have ranged from 100-150 l/m²/hr. The whole life cost evaluation
of capital and operating factors revealed better economies for the cross-flow
units, especially due to significantly reduced in-situ maintenance and membrane
To provide a full wastewater treatment and re-use facility, the AMBR process
is provided with upstream screening and balancing facilities and the treatment
is completed using reverse osmosis (RO) followed by UV disinfection to yield
potable quality water.
The upstream facilities comprise a 150m3/hr rotary drum screen providing screening
down to 0.75mm and a 900m3 balance tank offering 18hrs holding capacity. The
balance tank is aerated and mixed using two self-entraining Jetox aerators and
pH correction is provided to ensure the wastewater is suitable for feeding to
the bioreactors. A proportion of the wastewater collected at the balance tank
is discharged directly to the public sewer, having already undergone screening,
balancing and pH correction.
The remaining wastewater (up to 815m³/day of the total influent of
1,200m³/d) is passed forward to the bioreactors for biological treatment
in the AMBR process. The contents of the bioreactors (biomass/treated water)
are pumped continuously through the UF membrane systems. The biomass is retained
in the membrane tubes and is returned to the bioreactors. The treated water
passes through the membranes and is collected as permeate from the UF system.
At this stage the UF permeate is solids free, containing virtually no BOD and
only low residuals of COD (i.e. 5 to 25mg/l).
Up to 34m³/hr of the UF permeate is passed to a two-stage RO plant
arranged as a 3 x 2 array. The reject from the first stage is fed to the second
stage to increase the overall recovery to 75-80%. The RO system acts as a molecular
filter producing very high quality water (typically 30-50mS/cm² conductivity)
and a reject stream containing all the retained salts and any remaining organics.
The RO reject stream joins the sewer discharge flow from the balance tank and
up to 27m³/hr of RO permeate is produced as treated water.
The RO process prevents the passage of bacteria to the RO permeate but as an
additional precaution the treated water is pumped through a UV disinfection
unit which applies a ultraviolet dose of >38W/m². This high quality,
bacterially pure water is mixed with incoming town’s water to produce a water
suitable for re-use in the production processes carried out at site.
Commissioning of the plant began in February 2001 with the start-up of the
balancing facilities, the seeding of the first bioreactor and start-up of two
of the membrane banks. Development of the biomass to around 11,000mg/l was achieved
by mid-May at which point the second bioreactor was brought on-line. By the
end of May the RO system and UV disinfection unit were commissioned and potable
quality water was recycled back to the factory for the first time.
High rates of COD removal were achieved within a short time of bioreactor start-up.
By the start of April, when regular sampling and analysis was established on-site,
the COD removal was in excess of 98% with UF permeate COD concentrations consistently
below 15mg/l. Fig. 1 indicates the system’s ability to cope with variable feed
COD concentrations whilst maintaining very low and stable outlet COD concentrations.
The membrane separation process provides ‘filtered’ quality water and therefore
suspended solids in the permeate have been virtually undetectable (less than
Due to the final re-use, Kanes Foods pays particular attention to the quality
of the recycled water. Consistent monitoring of the final water product has
shown it to be completely clear of total coliforms and total bacteria at all
The membrane is outperforming the original design specification, with permeate
flux averaging around 135 l/m²/hr. Fig. 2 shows how the membrane flux
rate slowly deteriorates due to fouling and how it recovers following periodic
cleaning of the membranes. The cleaning frequency has been around one clean
every two months.
Overall, the Kanes plant has proved straightforward to operate and maintain.
The UF membrane separation provides the flexibility to independently manage
the hydraulic and solids retention times within the bioreactors, allowing a
high operating biomass population and long sludge age. These characteristics
enable the AMBR process to deliver a commercially attractive, compact and robust
water treatment and reuse solution
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