ABSTRACT
EXAMPLE 22.1 The land in a recent housing development was found, after the construction of the houses, to be unsuitable for septic tank usage, so cesspools were considered. The development had a persons equivalent of 58, so assuming a daily per capita water usage of 180 l the daily fl ow was:
180 58 1000
10 44 3
. m day 1
In this case the council was faced with three tank sizes, 31 m 3 to give a minimum retention time of 3 days to cover a weekend, 73 m 3 for 7 days ’ retention to cover the Christmas period or 475 m 3 for the recommended minimum 45 days ’ retention period. While the land proved ultimately unsuitable for the construction of such a tank, the removal of 3810 m 3 of sewage each year from the site using a 9 m 3 tanker would have required over 420 trips per annum and at an average cost (2010) of £ 50 per load, this would have amounted to £ 21 000 each year. The comparable cost of collecting and disposing of 3 m 3 of effl uent for a private individual would cost in excess of £ 200 per load. In this case the problem was resolved by the construction of a small package sewage treatment plant comprising two septic tanks, an RBC and a tertiary reed bed.
