Environmental Engineering FOURTH EDITION

Wastewater Treatment 169

Table 9-1. Characteristics of Typical Domestic Wastewater

Parameter

Typical value for

domestic sewage

BOD 250 mg/L

ss 220 mg/L

Organic and ammonia nitrogen 40 mg/L

PH 6.8

Chemical oxygen demand 500 mgL

total solids 270 mg/L

Phosphorus gmgn

ON-SITE WASTEWATER TREATMENT

The original on-site system, of course, is the pit privy, glorified in song and fable.’
The privy, still used in camps and temporary residences and in many less industrial-
ized countries, consists of a pit about 2 m (6 ft) deep into which human excrement is
deposited. When a pit fills up, it is covered and a new one is dug. The composting
toilet, which accepts both human excrement and food waste, and produces a useful
compost, is a logical extension of the pit privy. In a dwelling with a composting toilet,
wastewater from other sources, like washing, is discharged separately.
By far the greatest number of households with on-site disposal systems use a form
of the septic rank and tilejeld. As shown in Fig. 9-2, a septic tank consists of a concrete
box that removes the solids in the waste and promotes partial decomposition. The solid
particles settle out and eventually fill the tank, thus necessitating periodic cleaning.
The water ovefiows into a tile drain field that promotes the seepage of discharged
water.
A tile field consists of a plastic pipe with holes in it laid in a 3-fi-deep trench.
The effluent from the septic tank flows into the tile field pipes and seeps into the
ground through these holes. Alternatively, seepage pits consisting of gravel and sand
may be used for promoting absorption of effluent into the ground. The most important
consideration in designing a septic tank and tile field system is the ability of the ground
to absorb the effluent. Septic tank design includes the classification of soils that have
been found to “perc,” or allow the treated wastewater to percolate into the soil.

‘A literary work on this theme is “The Passing of the Backhouse” by James Whitcomb Riley:

But when the crust was on the snow and the sullen skies were gray,

In mth the building was no place where. one would wish to stay,

We did our duties promptly, there one purpose swayed the mind,

We tarried not nor lingered long on what we left behind.

The torture of that icy seat would make a Spartan sob;

(lines 25-29)