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Withy bed sewage treatment, Killerton House
Broadclyst, Exeter, Devon, UK
associated engineer
Not known
date  1778 - 1779, 2000
era  Georgian  |  category  Sewage/Sewer  |  reference  SS977002
Killerton House in Devon has a natural method for treating its sewage, based on the 18th century techniques used when the house was built. Willow saplings are used to absorb most of the nitrates, phosphates and ammonia in the raw sewage, decontaminating the effluent before it is flows into the local watercourse. The system was designed for the house and its occupants, and now deals with the waste produced by the 130,000 annual visitors to Killerton.
The Killerton estate covers 2,590ha on the east side of the M5 motorway. The Grade II* listed manor house and stable block were built in 1778-9 for Sir Thomas Dyke Acland (1722-85), whose family lived at Killerton House until it was donated to the National Trust in 1944 by Sir Richard Acland (1906-90). The house was designed by architect John Johnson (1738-1814), and was intended as a 'temporary' residence for the Aclands while a larger house was built elsewhere on the estate, plans that were later abandoned.
Horticulturalist John Veitch (17521839) created the gardens and was most probably also involved with the sewage treatment system. Originally, sewage was drained into a series of ponds, where the solids settled out, before the liquid effluent was discharged into a nearby stream. The ponds were later planted with willow saplings and turned into withy beds. At the time, sewage treatment usually consisted of land irrigation, sometimes using willow beds and dosing with lime to reduce smells.
The technique of using willow to purify raw sewage has been updated in modern times. Denmark and Sweden have been using willow treatment beds since the 1980s and 90s. The present withy bed system at Killerton was installed in around the year 2000, in the same location as the original beds.
Raw sewage from Killerton House flows through a 100mm diameter pipe to a connection with the 2m diameter main sewer and a 2m diameter branch from the stable block. The sewer conveys wastewater to a sealed underground septic tank, where anaerobic bacteria break down the sewage.
A 150mm diameter pipe takes effluent from the septic tank to a splitter box next to the first willow bed. The 5m wide splitter box reduces the flow rate from the pipe and creates an even distribution of effluent via trickle flow over a castellated lip running the full width of the withy bed. The willows absorb nitrates and ammonia (and water) from the effluent, while aerobic bacteria in the soil digest the organic matter with the help of the oxygen produced by the root system of the willows.
At the end of the first withy bed, a sluice chamber is used to sample the effluent and to activate controlled flooding. Occasional flooding eliminates unwanted grasses and weeds that can reduce the bed's effectiveness. The chamber's outlet pipe can swivel through 90 degrees to control water levels.
The sequence of splitter box, willow bed and sluice chamber is then repeated through a second withy bed. The effluent leaving the second bed is cleaner than from the first. It passes through a final splitter box and flows into a 'polishing' bed. This has a hardcore base to attract aerobic bacteria, and is planted with stinging nettles and reeds that remove trace elements. The treated effluent is then discharged through an outfall into the stream.
The Environment Agency makes regular checks on water quality. Studies show that willow trees can destroy an average of 60% of the harmful nitrogenous matter in sewage effluent.
Research: ECPK
"Treatment by willow-plantation or reed-bed zones at the outlet of a sewage treatment plant", Wikiwater, March 2012
"Reed Beds & Constructed Wetlands" by Louise Halestrap, Centre for Alternative Technology Publications, Machynlleth, August 1998, updated 2006

Withy bed sewage treatment, Killerton House