Table of Contents
Waste water treatment is the process of removing contaminants from waste water and house hold sewage to acceptable levels for release back to the environment. The aims of waste water treatment include:
- To protect public health;
- To convert waste materials present in waste waters into stable oxidized end products that can be safely disposed off to inland waters without any adverse ecological effects;
- To ensure waste water is effectively disposed off on a regular and reliable basis without nuisance or offence;
- To recycle and recover the valuable components of waste water such as biofuel, phosphates, coagulants, and carbon source;
- To protect in-stream water quality and the health of the organisms living in and around the receiving stream.
- To provide an economic method of disposal;
- To comply with legal standards and consent conditions placed on dischargers.
We shall consider waste water management in the city of Salem, Oregon State. It has an officially estimated population of 155,469 as of 2009 making it the third largest city in Marion County after Portland and Eugene. The city serves as a major hub for farming activities and is therefore a major agricultural production center. It has also attracted a number of major computer related manufacturing plants such as the Sumitomo Mitsubishi Silicon Group (SUMCO). Considering its economic and demographic factors, the city's major effluents include sewage waste, agricultural water waste and industrial water waste.
The City of Salem water waste treatment is managed by the Willow Water Pollution Control Facility (WLWPCF). The sewer collection system also includes Keizer, Turner and other incorporate areas of Marion County extending the service population to 225,000. The performance of the facility is measured through the collection and analysis of influent and effluent samples on a daily basis. The facility normally handles between 14 and 16 billion gallons of waste water annually.
The following table contains average waste water composition from the city of Salem.
|Dissolved solids (TDS)||1170|
|Nitrogen (as N)||150|
|Phosphorus (as P)||25|
|Alkalinity (as CaCO3)||850|
|Sulphate (as SO4)||90|
Design of a water treatment plant to suit the city of Salem
The water treatment plant should be able to reduce the level of contaminants from the city of Salem sewerage water to meet the following final consent parameters:
- 20 mg/l Suspended solids
- 10 mg/l BOD
- 3 mg/l Ammonia
- 2 mg/l P
WASTE WATER CONSTITUENTS REMOVAL OVERVIEW
Biochemical oxygen demand (BOD)
It is the measure of the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. It is based on the activity of bacteria and other aerobic mechanisms. A BOD test indicates the amount of water dissolved oxygen in milligrams per litre of oxygen. The higher the BOD, the higher the amount of pollution in the test sample.
In the septic tank, an anaerobic process takes place; some BOD is therefore removed by anaerobic digestion and by solids which settle to the bottom but much more of the BOD flows to the leaching field. Because BOD serves as a food source for microbes, BOD supports the growth of the microbial biomat which forms under the leaching field. This has its positives and negatives: on one hand the healthy biomat is desired because it is capable of removing many of the bacteria and viruses present in the waste water so that they do not pass to the ground water and digest most of the remaining BOD in the waste water. However, if the BOD is so high that all available oxygen consumed or if the leaching field is poorly aerated, the biomat can go anaerobic resulting in the death of desirable bacteria and protozoans in the biomat. Low oxygen could also encourage the growth of anaerobic bacteria which produce a mucilaginous coating which can quickly clog the leaching field making it ineffective.
BOD is removed from waste water during treatment by providing a supply of oxygen for aerobic bacteria which break down the organic BOD. It is worth noting that low BOG in waste water may result from ineffectiveness of biomat in the leaching field and that it acts as a food source for the denitrifying bacteria needed in bacterially-mediated nitrogen removal processes. Thus the denitrification process cannot operate effectively without enough BOD to support the growth of the bacteria which bring about the achievement of the process.
Water waste contains a large number of organic and inorganic suspended solids. They are expressed as miliigrams (mgs) per liter of water. Because they are mostly small, they are carried along with the waste water to the leach field and can cause its clogging. The septic tank effluent filter is used to reduce the suspended solids from waste water. It is fitted on the outlet tee of the septic tank and prevents suspended particles from flowing out of it. Other alternative methods include use of settling compartments and filters using sand or other media.
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Nitrogen present in the septic system from organic human waste is completed to ammonia by microorganisms in the septic tank. Ammonia is broken down to nitrates in the presence of oxygen and bacteria. IN a well aerated leaching field, the process takes place beneath it.
Nitrate and other forms of nitrogen are harmful to the environment, especially in coastal areas where excess nitrogen stimulates eutrophication. Alternatives have thus been sought to remove nitrogen from wastewater. Bacteria are used to convert ammonia and nitrate to gaseous nitrogen, N2 which is inert and released to the air.
Ammonia is first oxidized to nitrate which is then reduced to nitrogen gas. The reactions are carried out in different areas in the waste water management system.
NH4 + 3/2 O2 NO2- + 2H+ + H2O
NO2- + 1/2 O NO3-
(This process requires and consumes oxygen and is mediated by the bacteria Nitrosomonas and Nitrobacteria which require an aerobic environment).
The denitrification process can be summarized as:
NO3- + 5/6 CH3OH 1/2 N2 + 5/6 CO2 + 7/6 H2O + OH-
(The process occurs under anaerobic conditions; the bacteria present metabolize BOD as their food source)
Phosphorus is a constituent of domestic waste water principally found as organically bound phosphorus (body and food waste), polyphosphates (synthetic detergents) and orthophosphates.
Both organically bound phosphorus and polyphosphates can be hydrolyzed to orthophosphates and thus orthophosphates is considered the major component in waste water although other forms may still exist.
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Orthophosphates consist of PO43-, HPO42-, and H2PO4- which may form various chemical combinations with any available cations.
Phosphorus is removed under the leaching facility by chemical precipitation.
Orthophosphates combine with iron or aluminum ions to form the insoluble precipitates FePO4 and AlPO4 at a slightly acidic pH.
Fe3+ + (HnPO4)(3-n) FePO4 + nH+
Al3+ + (HnPO4)(3-n) AlPO4 + nH+
Wastewater contains trace amounts of iron and aluminum bind with phosphorus and thus cause some removal of phosphorus below the leaching facility. The best method of maximizing phosphorus removal is to locate the leaching facility above the ground water to prevent chemical reduction of iron to Fe2+ which is soluble and travels with ground water.
WASTE WATER TREATMENT PROCESS
Waste water undergoes the following main stages before release to the environment.
- Preliminary treatment
- Primary treatment
- Secondary treatment
- Final (Tertiary) treatment
Preliminary treatment of wastewater includes screening, of large objects, grit removal and flocculation. It is an important step as it helps protect equipment on other stages of the treatment process. Air flotation and flocculation aid in the removal of suspended solids and oil in the primary clarifier and reduce the biological loading on secondary treatment processes. Prechlorination or pre-aeration may be required to prevent odor problems and to eliminate septic conditions where wastewater has abnormally long runs to the plant. Equalization structures are used to dampen diurnal flow variations and to equalize flows to treatment facilities.
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Their primary function is coarse screening for protection of downstream equipment.
Screens will be located where they are readily accessible. An approach velocity of 2.0 feet
per second, based on average flow of wastewater through the open area.
Bar spacing: Clear openings of 1 inch will be used and 5/16-inch x 2-inch bars up to 6 feet in length and 3/8-inch x 2-inch or 3/8-inch x 2½-inch bars up to 12 feet in length. The bar will be long enough to extend above the maximum sewage level by at least 9 inches.
Size of screen channel. The maximum velocity through the screen bars, based on maximum normal daily flow, will be 2.0 to 3.0 feet per second during wet weather flows or periods of emergency weather (On the basis of the screen being entirely free from debris). Knowing the maximum storm flow and the maximum daily normal flow, the channel size is determined as follows: the sewage flow (million gallons per day) multiplied by the factor 1.547 to get the sewage flow in cubic feet per second which when divided by the efficiency factor obtained from table 10-1 will give the wet area required for the screen channel. The width of the channel should therefore range from 2-4 feet. The sewage in the screen channel should be kept as shallow as possible as to keep down the head loss through the plant.