Juanico, consulting consultants, municipal sewage treatment, wastewater effluents, CEPT, Chemically Enhanced Primary Treatment, APT, Advanced Primary Treatment, coagulation, flocculation. Marine outfall.

HOME

CEPT - Chemically Enhanced Primary Treatment

(also called APT - Advanced Primary Treatment)

Municipal sewage treatment: the conventional approach

A conventional sewage treatment plant is made of a series of different treatment steps:

Screening
Sand removal (with coupled oil and grease removal if necessary)
Primary sedimentation
Biological reactor
Secondary sedimentation
Post-treatment if necessary (filtration, disinfection, etc.)

Municipal sewage treatment: the CEPT conception

The CEPT proposal is to enhance primary sedimentation by the use of iron and aluminum salts, lime and polyelectrolytes, in order to increase the coagulation, flocculation and sedimentation of raw sewage.

This process drastically increases the performance of primary sedimentation (higher removal of TSS, BOD, nutrients, etc.) without a large increase in the amount of sludge produced.

The resulting effluents can be:

Discharged directly to the environment if the proper conditions are present (e.g., in the case of a marine outfall)
Treated in a biological reactor; in this case the reactor will be smaller with lower construction and operational costs

CEPT is an old technology (1740 in France) but the development of biological reactors stopped its development and use for decades. Now it revives due to the development of better coagulants and flocculation enhancers, stricter environmental standards and the need for low-energy treatment technologies.

CEPT can be applied both to small cities and huge mega-cities.
CEPT is an ideal solution for tourist resorts and coastal cities where population abruptly increases during the high-season. CEPT has higher flexibility than any biological reactor.

There are already numerous applications of CEPT around the world.
The two most famous cases are:

Los Angeles - USA - Hyperon Treatment Plant (4 million inhabitants)
Hong Kong (6 million inhabitants)

CEPT Performance

Coagulants' typical doses are between 10 and 50 mg/l depending on several factors.
Flocculation enhancers' typical doses are between 0.3 and 1 mg/l (anionic or cationic polyelectrolytes).

Typical CEPT removal efficiencies are:

Parameter	Removal %
TSS	70 - 90
BOD5	50 - 65
COD	55 - 75
Nitrogen	50
Phosphorus	50 - 85
Faecal coliforms	99.9
Helminth ova	almost complete

The concentration of TSS in the treated effluent is lower than 30 mg/l, low enough to allow a proper disinfection efficiency by UV.
The combination of this fact with the new UV disinfection technologies in the market, makes the CEPT-UV couple a good treatment alternative.

Sludge production increases only by 5 - 15 %.

Coagulation/flocculation: the need for optimization

TSS in sewage use to have the same electrical charge and this limits the formation of large particles with good settling characteristics.

Coagulation is, basically, the neutralization of the electrical charges of the particles, allowing the formation of larger particles (flocs) and thus enhancing settling.

Most common coagulants are:

ferric chloride
aluminium sulfate (alum)
lime

Flocculation enhancers are long-chain polyelectrolytes that "catch" small particles into big ones.

In most cases the electrical charge of particles in municipal sewage is negative.
But - warning !- this is not a 100% condition. Rare ionic composition of supply water or heavy contribution of some industrial effluents to municipal sewage may change the situation. Actual composition of raw sewage may be very complex.

Thus, anionic polyelectrolytes may result more efficient than cationic ones and -in some extreme cases- even the coagulation salts must be changed.

The coagulants, enhancers and optimal doses are different for each project, and they may even change in the same treatment plant during the year (summer-winter, rainy-dry season, etc.). Thus, a successful CEPT operation requires frequent analyses for optimization of the coagulation process, including chemicals, doses, pH, mixing, etc.