It takes both primary treatment and secondary treatment combined to make a properly designed on-site sewage disposal system.
Primary Treatmenti s the initial process in which the solids are removed from the liquids through settling. The two types of primary treatment are the septic tank and the equalization tank for the aerobic system. Primary treatment involves the following activities:
Screening- to remove large objects, such as stones or sticks, which could plug lines or block tank inlets.
Grit chamber- slows down the flow to allow grit to fall out, and
Sedimentation tank (settling tank or clarifier)- settle able solids settle out and are pumped away, while oils float to the top and are skimmed off
The household type of septic tank has been discussed in the Unit 1 of this module. Globally, septic tanks are the most common primary treatment method used for onsite sewage disposal systems. Septic tanks can be made of pre-cast concrete, fiberglass or plastic.
Septic tanks must have a minimum capacity of 1,000 gallons. They are made in different shapes and sizes to accommodate different sites. The septic tank receives household wastewater and helps the liquid wastes to separate from the solid wastes.
The heavier solids settle to the bottom while the lighter solids (greases, etc.) float to the surface of the tank. The liquids (effluent) from the septic tank then discharge into a secondary treatment system.
Equalization tanks provide the primary treatment for wastewater entering aerobic systems with septic tanks. The job of the equalization tank is to separate `the solids from the liquids. It also equalizes the strength of the sewage entering the aerobic treatment unit. Equalization tanks must have a minimum capacity of 300 gallons and maximum capacity of 1,000 gallons.
Secondary Treatment is the biological breakdown of the remaining organic matter in the effluent after primary treatment. Secondary treatment reduces the pollutants and pathogens in the effluent prior to disposal.
Read Also : Solid Waste Management Complete Guide
There are several types of secondary treatment systems available for use: Subsurface Absorption Fields, ETA Systems, Lagoons, Aerobic Systems and other approved alternative systems including Constructed Wetlands.
Subsurface absorption fields
The most common on-site wastewater disposal method is the subsurface absorption field. It consists of a network of shallow trenches filled with an absorption media (usually rock gravel or other similar material).
The gravel surrounds perforated pipe and is covered with the excavated native soil. Effluent from the septic tank slowly trickles through the pipes into the absorption media where it is stored. The effluent is gradually absorbed into the soil.
The effluent is filtered and cleansed by the soil and natural bacteria. This type of disposal is totally dependent on the soil’s permeability (its ability to absorb water). Subsurface absorption fields do not work well in tight or high clay content soils.
Evaporation transpiration/absorption (ETA) systems
ETA systems are generally used in tight clay soil. The trench construction is the same as the subsurface absorption field with the exception of the backfill material. Instead of being used as backfill, the excavated native soil (clay) must be removed.
The trench is back filled with sand instead of the clay and then capped with two to four inches of new topsoil. The wastewater wicks upward through the sand where it can evaporate and/or be utilised by vegetation.
Like the typical subsurface absorption field, ETA systems are sized according to water usage and location. However, they are required to be much larger than subsurface absorption fields. The use of ETA system is not recommended in areas of high rainfall and low evaporation.
A lagoon is a shallow total retention, pond-like structure in which the effluent is exposed to sunlight and oxygen that enables aerobic bacteria to digest the organic matter. Lagoons are sized based upon local rainfall and evaporation rates.
A properly sized lagoon will have no discharge to the environment since the volume of the wastewater is reduced through evaporation. This type of system is very effective and relatively easy to maintain; however, it requires a large lot or site for construction.
Aerobic systems with surface application
Aerobic systems consist of an aerobic treatment unit with surface application of the treated effluent. Aerobic treatment units (see Figure 19) are used primarily when environmental conditions preclude the use of subsurface absorption fields. Treatment occurs when air is injected into the wastewater to promote biological breakdown of the organic matter.
Chlorination completes the treatment prior to disposal. While for most on-site sewage disposal systems the secondary treatment also accomplishes disposal, this is not true for aerobic systems. The aerobic treatment process produces a high quality effluent that still requires disposal.
This high quality effluent may be applied directly to lawns through a timed sprinkler system that sprays the effluent over a large vegetated area at night. Most of this water will evaporate and the rest will be absorbed and utilized by the vegetatio
Secondary treatment typically utilizes biological treatment processes, in which microorganisms convert non-settleable solids to settleable solids. Sedimentation typically follows, allowing the settle able solids to settle out. Three options include:
Activated Sludge- The most common option uses microorganisms in the treatment process to break down organic material with aeration and agitation, then allows solids to settle out. Bacteria-containing “activated sludge” is continually re- circulated back to the aeration basin to increase the rate of organic decomposition.
Trickling Filters- These are beds of coarse media (often stones or plastic) 3-10 ft. deep. Wastewater is sprayed into the air (aeration), and then allowed to trickle through the media.
Microorganisms attached to and growing on the media, break down organic material in the wastewater. Trickling filters drain at the bottom; the wastewater is collected and then undergoes sedimentation.
Lagoons- These are slow, cheap, and relatively inefficient, but can be used for various types of wastewater. They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).
After primary and secondary treatment, municipal wastewater is usually disinfected using chlorine (or other disinfecting compounds, or occasionally ozone or ultraviolet light). An increasing number of wastewater facilities also employ tertiary treatment, often using advanced treatment methods.
Tertiary treatment may include processes to remove nutrients such as nitrogen and phosphorus, and carbon adsorption to remove chemicals.
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These processes can be physical, biological, or chemical. Settled solids (sludge) from primary treatment and secondary treatment settling tanks are given further treatment and undergo several options for disposal.
Below is a sketch diagram of a centralized municipal wastewater collection and treatment system (Figure 21). I advise that you properly study it and take your jottings. I am sure it will help you understand more of wastewater collection and passage.
Fig. 21: A View of a Centralized Municipal Wastewater Treatment System