The stipulations regarding the Porlock plant were that it should be a small-footprint plant, and able to disinfect the sewage whilst imposing little visible impact. The plant therefore had to be housed in a stone-faced building identical in appearance to an adjacent farmhouse.
It comprises four aeration tanks, 89 m3 in liquid volume, with each tank contains six membrane units with 150 panels per unit giving a total membrane panel area of 2880 m2 provided by 3600 membrane panels.
Pretreatment comprises a 3 mm perforated screen, this having replaced the original 2 mm wedgewire screen. At a peak flow of 1.9 MLD, the plant operates at a flux of 27 LMH, but the average flux is generally below 20 LMH and the TMP usually between 0.02 and 0.11 bar under gravity-feed conditions (governed by the tank depth and pipework hydraulic losses). The membrane module is aerated at a rate of 0.75 Nm3 hr-1 per m2 membrane area. The membrane is relaxed for 30-60 minutes daily as a result of low overnight flows, and cleaning in place with 0.5 wt% hypochlorite is undertaken every 8-9 months.
Diffusers are flushed manually without air scour. The plant operates at SRTs of 30-60 days and the recommended MLSS concentration range is 12-18 g/L, but it has been operated as high as 30g/L and is regularly over 20 g/L.
The plant is of a simple and robust construction, and has generally proved very reliable. Membrane replacement due to actual membrane failure for this plant was minimal prior to substantial replenishment (60%) after 12 years; Porlock was originally costed on the basis of a membrane life of seven years. Periodic fouling events have been encountered at this plant, and some of these have been linked with seawater intrusion.
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