Sewage Works Design Small Treatment Plants

Small treatment plants typically discharge treated effluents to a surface water body as contrasted to septic systems, which discharge to groundwater systems. Surface discharges require a permit from a regulatory agency and allowable effluents are based on the minimum average 7-day flow expected to recur once in 10 years (MA7CD10), upstream and downstream discharges, and downstream uses. Some of the more common flow diagrams for small sewage treatment plants are illustrated in Figure 3.12 and predesigned and prefabricated units are available.

As noted in Figure 3.12, bar screens or comminutors and grit chambers are provided ahead of pumping equipment and settling tanks to remove larger solids. If secondary treatment is needed, primary treatment units should be designed to have water level of sufficient height to permit gravity flow to the both the secondary units and to the receiving stream without additional pumping.

Location of small treatment plants should take into consideration the type of plant desired, the availability of supervision, the location of the nearest residence, the receiving watercourse, the likelihood of flooding, prevailing winds and natural barriers, and the cost of land. A distance of 400 feet from the nearest residence is frequently recommended, although distances of 250 to 300 feet should prove adequate with good plant supervision. Oxidation ponds and lagoons should be located at least 0.25 to 0.5 mile from the nearest human habitation.

Disinfection

The need for disinfection of sewage effluent depends on the probability of disease transmission by the ingestion of contaminated water or food including shellfish, by contact, and by aerosols.

This probability should be balanced against the effects that chlorination can have on aquatic life.35 Normal chlorination does not destroy all pathogenic viruses, fungi, bacteria, protozoa, and helminths. Although chlorine as a disinfectant is discussed here, it does not preclude dechlorination or the use of other disinfectants. Also, wastewater must be adequately treated in the first place for the disinfectant (usually chlorine) to be effective.

Chlorination treatment of raw sewage is not reliable for the destruction of pathogenic organisms since solid penetration is limited. The dosage of chlorine required to produce a 0.5 mg/l residual after 15-minute contact has been estimated for different kinds of sewage (see Table 3.13). An early study found that less than 250 coliform organisms per 100 ml remained in treated sewage if a chlorine residual of 2.0 to 4 mg/l is maintained in the effluent after 10-minute contact.36 Other tests found that with no mixing, at least twice the chlorine residual had to be maintained in the treated sewage for 10 minutes to get results approximately equal to those obtained with mixing.

FIGURE 3.12 Typical flow diagrams.

^ TABLE 3.13 Probable Chlorine Dosages to Give a Residual of at Least 0.5 mg/l after 15-Min Retention in Average Sanitary Sewage or Sewage Effluent"

Type of Sewage Effluent

N.Y. Dunham"

State""

Whitef

Suggested Chlorine Dosages, (mg/l)6 Griffin«'1

Imhoff and Fair1

Metcalf & GLUMRBJ Eddy'

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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