Membranes boost water re-use

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.

Test facility

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

the factory.

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

cleaning/fouling considerations.

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

10mg/l).

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

times.

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