MUNICIPAL WASTEWATER 735
plant stream. At the end of the cycle the dewatered sludge is
removed by a scraper.
Dried sludge can be incinerated, taken to a landfill dis-
posal site, composed or subjected to superoxidation. Some
coastal cities barged sludge to sea. This was to have ceased
in 1981 but was permitted to continue for ten years. It is not
allowed after 1991. Incineration of sludge has the potential
for air pollution problems and often there is local opposition
to installation of an incinerator. Incinerators are expensive to
build and operate. Land disposal is expensive and disposal
sites frequently are considerable distances from the gener-
ating wastewater treatment plants. Groundwater pollution
can occur. Landfills which can accept digested sludge are
in short supply. Transportation costs are a quite significant
part of total disposal costs. Composting has been suggested
as a possible ultimate solution. The requirement of relatively
large land areas and odor production are problems.
A promising approach involves superoxidation. Here the
sludge is treated with a strong oxidizing agent. Volume is
reduced greatly and the end product is stable and inoffen-
sive. Transportation costs are thus reduced.
It is common practice to chlorinate effluent for bacte-
rial control. Regulations vary from state to state, but most
regulations require chlorination to a specified residual.
Requirements usually vary from season to season, the most
stringent rules governing the swimming season.
A phenomena not yet fully understood is that of after-
growth, wherein bacterial count is fairly low immediately
after effluent discharge but then suddenly rises to a high
figure.
In some plants chlorination of the influent is practiced
for the purpose of odor control.
Chlorination of storm water overflow is commonly prac-
ticed. In some cases storm water overflow is subjected to
simple sedimentation and/or screening, storage and chlori-
nation, then discharged after cessation of the storm.
One commonly used definition of tertiary treatment is any
treatment in addition to secondary (biological) treatment.
Tertiary treatment is practiced when an effluent of much
higher quality is required than is attainable with conventional
biological treatment. They type process used will depend on
the final effluent quality necessary and the economics of the
total process.
Commonly used tertiary treatment processes are listed
below:1) Sand filtration
2) Microstrainers
3) Oxidation ponds
4) Foam separation
5) Activated carbon adsorption
6) Chemical clarification and precipitation
7) Ion exchangeDisposal of human and kitchen wastes in areas not served
by sewers and wastewater treatment plants presents unique
problems.Disposal must be in the immediate vicinity of the source
of the wastes. In adequate controls are not exercised a closed
system may results. An example is Suffolk County (Long
Island), New York. Septic tanks are widely used, and there
is strong local opposition to the considerable expense of
installing sewers. Effluent from the septic tanks found its
way into the ground water which is the supply for much of
the county. Eventually, the problem was graphically pointed
up by the appearance of foaming detergents in water issuing
from the tap.
In more primitive societies waste disposal is a matter of
convenience. A “cat hole” or communal straddle trench is
utilized and covered when capacity is reached. This is the
same as the practice with privies and cesspools.
Privies, as the name implies, are simply open pits with
a structure to provide privacy. Human excreta is deposited
into the pit and is slowly stabilized. Stabilization is slow,
due principally to the presence of urine. Pits may be open
earth or concrete vault. Drawbacks for both types are odors
and fly problems. For the unlined pit there is the additional
problem of ground water pollution. Older privy construction
allowed access to flies around the edge of the pit. For proper
protection against flies there must be a tight seal around the
edge of the pit and adequate screening of openings in the
privy structure itself.
In the 1930s a large number of the older privies were
replaced by concrete vault types. Today, such methods of
waste disposal are found only in the smaller rural communi-
ties where there is no municipal collection system.
Cesspools are simply pits into which waste is allowed to
flow. The term leaching pit is sometimes used. Water seeps
into the ground, leaving solid matter in the pit. Construction
is of two types. A pit may be unlined, or it may be lined with
sewer pipe laid on end. Almost nowhere in the United States
are cesspools permitted by health authorities.
Septic tanks are widely used in smaller towns and out-
lying suburbs of larger cities. They are a combination sedi-
mentation tank and anaerobic digester. Sanitary and kitchen
wastes flow into the tank and grease and light material rise to
the top. Heavier particles settle to the bottom where anaerobic
stabilization occurs. Deflector plates are provided at inlet and
outlet in order to minimize short circuiting. Effluent flows to
a tile field where disposal is into the earth. The tile field is
composed of perforated field tile fed by a manifold. The tile is
underlain with granular material, usually gravel. Care must be
taken that the earth does not become clogged by material car-
ried over from the septic tank. Septic tanks are being replaced
as more and more areas are served by municipal systems.
Health authorities do not look with favor on septic tanks.
Capacity will be a function basically of the number of per-
sons or units served. Some experts feel that, in no case, should
capacity be less than 1500 gallons. Lesser volumes are permit-
ted in many codes and the thought that 1500 gallons ought to
be the minimum permitted arouses home builders and land
developers. Periodically it is necessary to employ a scavenger
service for emptying the tank of accumulated solids. Solids
thus collected may be discharged to a convenient treatment
system or directly to a wastewater treatment plant.C013_007_r03.indd 735C013_007_r03.indd 735 11/18/2005 10:43:42 AM11/18/2005 10:43:42 AM