Treatment of seasonal wastewater flows in a two-pond system

• First use of the light-dark technique to indicate WSP metabolic status.
• Higher than expected loading could be achieved on the first pond.
• Short periods of low light intensity can affect system performance.
• Start-up without acclimatisation achieved by retaining treated effluent over winter.
• Pond effluent could meet BOD and COD discharge standards to inland water.

A waste stabilisation pond system comprising two ponds was used to treat a seasonal discharge from a summer campsite in the UK. Despite a short retention time and relatively high surface loading, the first pond was able to acclimate rapidly to the incoming wastewater, although dissolved oxygen (DO) concentrations remained low. Successful operation was due in part to initial dilution of incoming load at the start of the season by treated water retained over the winter period. The two ponds in series produced a final effluent which met typical chemical and biochemical oxygen demand standards for discharge to inland waters; as expected nutrient concentrations were above typical limit values, making land application for irrigation a preferred option. The system performance was adversely affected by short periods (3–4 days) of low average light intensity, leading to reduced chlorophyll-a and DO concentrations which did not immediately respond to subsequent increases in irradiance. A light-dark bottle technique, adapted to determine net oxygen production potentials under standard conditions of illumination, mixing and temperature, was found to provide an excellent indicator of pond metabolic status and treatment potential.