Industrial Effluent Treatment Project

J.E. Hartley

The contract for the turnkey design, supply and construction of an activated sludge vegetable processing industrial effluent treatment system was awarded to Naston Ltd as main contractor by J.E. Hartley of Thorganby near York. Awarded in September 1999, the L1.4 million contract was driven by the need for the plant to be ready to treat pea processing effluent at the start of the next season in late June 2000. This target was achieved by carrying out the project on a fast-track basis.

The new plant was designed around the Sequence Batch Reactor (SBR) system, ensuring flexibility for an effluent stream that was known to be highly variable in terms of flow and biological load. The basis of the system was a pre-SBR balancing tank sized to accommodate the peak flows of effluent which occur in the summer season, whilst the dual reactors were designed for continued effective operation during the lower production periods of winter and spring, being able to operate on lower batch volumes and/or with one of the reactors idle.

Industrial Effluent Treatment Process
Industrial effluent from vegetable processing varies with the crop and less predictable factors including weather. One of the key design parameters therefore was flexibility. The plant had to be designed to treat raw effluent having 3000 - 7000 mg/I of BOD to a discharge consent quality of <20 mg/I BOD and <30 mg/l TSS. The installed system incorporates the following elements:

- Caustic soda and nutrient dosing to optimise the acidic, low Nitrogen effluent.

- Main pump station centralising forward and return flows.

- Aerated pre-treatment balancing tank, permitting up to 10 days retention at peak flow.

- Dual activated sludge SBR unit, utilising fine bubble diffusers with floating decanter mechanisms.

- Aerobic sludge digester, using coarse bubble aeration with overflow return of supernatant liquors.

- Post-treatment balancing to minimise effluent flow rates to outlet.

- Final effluent polishing, using coarse sand filtration.

- Fully integrated SCADA and PLC control system, with inlet and outlet flow and quality control monitoring.

Mixing, pH adjustment & Nutrient Adjustment
Caustic soda for pH adjustment and Purisol for Nitrogen and Phosphorus correction are dosed into a flooded mixing tank to combine with the screened raw effluent. The system includes bulk storage tanks and skid mounted dosing rigs. On-line monitoring of pH, COD, TSS and Ammonia provide a log of incoming effluent characteristics.

Sequencing Batch Reactor Treatment
The balanced effluent is pumped into the two SBR reactors, with staggered operation of 9 - 12 hour intervals. The mixed liquor is aerated using fine bubble diffusers, with optimum aeration controlled by dissolved oxygen probes. A submersible mixer maintains the homogeneity of the liquor between aeration cycles. The blower/mixer combination ensures very efficient operation. Filling of the reactors is carried out under anoxic conditions to control bacterial selection. Discharge of treated effluent is by floating decanter mechanism once every 24 hour cycle. Sludge is pumped from the reactors to the sludge digester.

Final Effluent Storage & Polishing
A post-SBR balancing tank is used to regulate flows passing to the outlet, so ensuring compliance with the discharge consent. A continuously backwashing coarse sand filter polishes all effluent flows and provides further security to final effluent quality. A final effluent monitor controls the outlet valve, which will divert any out-of-consent effluent back for further treatment.