Sanitary wastewater treatment
Sanitary sewage, which is also called domestic sewage or small communities, is the sewage that is produced from residential units, hospitals, commercial units, institutions and similar places. Sanitary waste water from daily human consumption includes internal uses such as bathroom, toilet, cooking, etc. Each community’s sewage is obtained from that community’s consumed water. Therefore, it is necessary to estimate that part of the used water that reaches the sewage network. In general, in order to determine the average amount of produced wastewater used in the project, it is necessary to estimate the household consumption per capita in the city. According to the standard of the country’s water industry, the per capita production of household wastewater in Iran is 75 to 150 liters per day, which can be used as a figure of 150 liters per day per person in the sewage network plan. Also, this amount must be added leakage percentage that refers to the chain wastewater piping age. The implementation of domestic wastewater treatment in small and large urban communities is important because it provides the possibility of reusing purified and sanitary water for every human community in order to reduce the spread of diseases that are transmitted through water and reduces the effects of on the environment.
Therefore, sanitary sewage pollutants are dangerous to the environment and must be treated and removed before disposal in the environment. Wastewater treatment standards are related to the separation of biodegradable organic matter, suspended solids and pathogens. Sanitary (domestic) wastewater comes from two primary sources: the wastewater caused by human daily consumption for cooking and washing dishes, bathing and washing clothes, and it is called gray water, and the wastewater in the bathroom and toilet that contains feces and urine. It is human and it is called black water.
Sanitary wastewater characteristics:
According to the characteristics of wastewater and the goals of sanitary wastewater treatment, can help us which systems and processes should be used for wastewater treatment. Therefore, first of all, the qualitative characteristics of wastewater should be obtained by testing wastewater samples. The quality characteristics of wastewater are:
- Physical characteristics
- Chemical characteristics
- Biological characteristics
Physical characteristics of wastewater:
The most important physical characteristics of wastewater include total suspended and dissolved solids, odour, temperature, density, and colour. The colour of sewage helps in visual and immediate recognition of the nature of sewage. Fresh wastewater usually has a light grey-brown colour, but as time increases and the dissolved oxygen concentration decreases in sanitary wastewater, the colour of wastewater changes from grey to dark grey and finally to black. In this situation, it is called “septic” or “rotten” sewage.
Chemical characteristics of wastewater:
The chemical characteristics of wastewater are related to organic substances, inorganic substances and gases in it. The compounds of organic matter in sanitary and industrial wastewater are usually a combination of carbon, hydrogen and oxygen and in some cases together with nitrogen. Wastewater organic matter is measured in different ways. The most important measurement methods are:
DO dissolved oxygen
Oxygen is dissolved in water, which is needed by aquatic plants and animals. DO should be at least 5 ppm for fish to live. Cold water has more DO.
BOD (Biochemical (biological) Oxygen Demand)
It is the amount of oxygen that aerobic microorganisms need to break down biodegradable organic materials into final products. This parameter depends on temperature and time. BOD is the amount of oxygen required for biological stability in water. The size of biological wastewater treatment facilities, especially the amount of wastewater aeration in aeration ponds, can be calculated based on BOD. The BOD of treated wastewater for discharge to the surface and underground water should be less than 20 mg/litre.
Chemical Oxygen Demand COD (Chemical Oxygen Demand)
In this test, the amount of oxygen that is chemically required to oxidize organic materials and convert them into CO2 and water is measured. For this purpose, a strong chemical oxidizing agent (such as potassium dichromate) is used in an acidic environment. A ferrous sulfate catalyst is used to help the oxidation process. The COD
test is used to measure organic substances in industrial wastewater and sanitary wastewater that contain toxic compounds for living organisms. The COD of wastewater is always higher than its BOD because more compounds are chemically oxidized, and this parameter is actually a measure of degradable and non-degradable organic matter.
Chemical tests are used to determine parameters such as PH- TDS- TSS- DO- BOD- COD, etc.
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The biological characteristics of wastewater, which are important in wastewater treatment, are discussed under three general headings:
- Group of biological purifying microorganisms
- Pathogenic microorganisms in wastewater
Indicator organisms of pollution and their identification and counting methods
Microorganisms that are important in wastewater treatment include bacteria, fungi, algae, protozoa, plants, animals, and viruses. The purpose of the disinfection process in sanitary wastewater treatment is to kill or neutralize these pathogenic microorganisms.
Sanitary wastewater treatment steps:
In general, sanitary wastewater is treated through the following three stages:
In the first stage of sanitary wastewater treatment, about 40-60% of suspended solid particles are removed. At this stage, some kinds of bar screens used to separate large and heavy floating objects. Bar screen, drum screen and band screen can be used in the screening process. Also, in order to separate heavy inorganic particles such as sand and pebbles and heavy granular materials such as corn kernels and body remains of organisms, a grit tank should be used, which consists of two types of constant velocity channels and continuous aeration channels (Aerated design). The reasons for using these two processes are to protect the mechanical equipment against wear and tear and prevent clogging of the pipes and reduce the accumulation in the equalization tank and other basins. After these two processes, we will have the primary sedimentation tanks where the particles settle due to the force of their gravity. In this process, we need a relatively high dwell time.
The purpose of this stage is the biological treatment of the effluent from the primary treatment by aerobic and anaerobic microorganisms. At this stage, more than 90% of organic materials are decomposed. In this purification, two processes of suspended growth or attached growth can be used, the most common of which is the use of suspended growth or activated sludge. In the activated sludge process, oxygen dissolved in water is used to transfer organic pollutants from dissolved to suspended solids (Bio Mass). The sludge settled in this process returns to an aeration tank for keeping the microorganism’s population. The excess sludge after the concentration and digestion processes will be used as fertilizer for agricultural purposes. In general, in this process, suspended solids and colloids that were not separated in the initial sedimentation are separated. Finally, more than 95% of organic pollutants are removed in this step.
If the water coming out of secondary wastewater treatment does not have the desired quality, advanced wastewater treatment should be used. The purpose of this treatment is to separate components such as nutrients, toxic compounds and large amounts of organic matter and suspended solids. The removal of inorganic substances (mineral salts) by two methods of ion exchange and semi-permeable membranes such as reverse osmosis, as well as the removal of organic substances by two methods of surface absorption (usually using activated carbon) and chemical oxidation, are included in this stage of purification. Finally, we have the disinfection stage using chlorine, ozone, and UV radiation to remove pathogenic and disease-causing microorganisms in sanitary wastewater.
Sanitary wastewater treatment package:
Providing sanitary water is a basic need that affects people’s standard of living. When sanitary sewage from small communities enters the environment without any treatment, it will pose risks to human health. On the other hand, in many areas, it is not possible to transfer sanitary wastewater to central urban treatment plants, or it will be expensive. Therefore, the use of sanitary wastewater treatment packages is affordable and in many cases, the water needed in the region for irrigation of agriculture or green space and even reuse in homes will be possible. A decentralized wastewater treatment system or wastewater treatment package will be a combination of a septic tank, an anaerobic reactor, and an aerobic process. To make it cost-effective, recycling and reusing wastewater at the point of production can solve water problems to a great extent. When one or more operating units are included in a single system for wastewater treatment, it is known as a package wastewater treatment system.
- Fat removal
- Aerobic biological treatment
Rapid advances in wastewater treatment engineering in the world have introduced human and sanitary wastewater treatment methods in a small volume and with low process-disposal sludge production.
Pajoohesh Sanat- Ab Company (Pasenco) introduces polyethylene septic tanks and an advanced treatment system to its valued customers in the majority of polyethylene and metal packages.
Sanitary and industrial wastewater treatment packages have special capabilities compared to existing wastewater treatment packages, both in terms of occupancy level and in terms of performance and cost.
- ندافی، کاظم: تصفیه فاضلاب، سازمان سازندگی و آموزش وزارت نیرو، تهران، چاپ اول، 1379