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.

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


Tags



Topics


Click a keyword to see more stories on that topic, view related news, or find more related items.

Comments

You need to be logged in to make a comment. Don't have an account? Set one up right now in seconds!


© Faversham House Group Ltd 2001. edie news articles may be copied or forwarded for individual use only. No other reproduction or distribution is permitted without prior written consent.