Effluent from which large debris and grits are removed after Preliminary Treatment of Wastewater is directed toward Primary Treatment of Wastewater. In general the wastewater contains 99.9% of water and 0.1% of solids in the form of dissolved, suspended and settle-able. Although the solids present are only 0.1% but if left untreated would cause serious hazard to the water bodies in which it will be discharged. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Effluent which leaves primary section is called as primary effluent.
In the primary treatment section, the wastewater flows through large settling tanks/ clarifiers, where almost 40-50% of the solids are removed by the means of settling. This is done by reducing the fluid velocity and increasing Hydraulic Retention Time. The settled solids form sludge which can be treated using various other techniques. During primary treatment of wastewater, lighter materials such as oil & grease float onto the surface can be removed using skimming and can be treated separately.
Primary treatment of wastewater involves removal of following impurities:
- Organic Matter: Solid Organic Waste, Toilet Waste, Paper etc. Food Processing, Textile Processing units contribute to large volume of organic material.
- Grease and Oil: Cooking oil, fat, soap and body oil from bath.
- Inorganic Material: Sand and Silt etc.
- Nutrient: Like Phosphorous, Nitrogen from animal waste.
- Metals: From Industrial Waste.
- Chemicals: Dish washing agent such as detergent, shampoo etc.
- Micro-organism: Usually from animal guts.
Some of the treatment units used in Preliminary Treatment are used at this stage with or without addition of chemicals. Sometimes preliminary treatment and primary treatment are collectively called as primary treatment section.
Units in primary treatment section:
1. Primary Sedimentation Tank
Since settling of effluent at this stage involves many unknown parameters for light weigh, sticky and non-regular shaped particles, the classical laws of sedimentation used in grit settling stage are not applicable. The settling type used at this stage is called as flocculant settling. The primary sedimentation tank usually removes 30 to 40% of the total BOD and 50 to 70% of the suspended solids from wastewater. However the actual efficiency depend upon wastewater characteristics and proportion of organics present. Detention period kept at this stage varies from 90 to 150 minutes. Mechanical collectors are provided to remove the collected sludge at bottom and skimming equipments are provided to remove floatable substances such as scum, oils and grease.
2. Equalization Tank/ Storage and Homogenization Tank
In centralized treatment facilities where variation in characteristics of effluent flow received is very pronounced Equalization Tank or Storage and Homogenization Tank are used. These are built to avoid shock loading to the system. Chemicals are dosed to prevent escaping of toxic gases and to avoid bad odor.
3. Skimming Tanks
Separate skimming tanks are required if the influent has more amount of floating impurities such as oil and grease, soap, pieces of cork and wood etc. These can be rectangular or circular with a detention period of 1 to 15 minutes.
4. Pre-Aeration Tank
Pre-Aeration of the effluent is achieved by introducing air into the wastewater for a period of 20 to 30 minutes at design flow. This is done to remove suspended solids in sedimentation tank, removal of grease and oil, to freshen up septic wastewater and BOD reduction.