Sewage treatment plant (STP) plays vital role in the process of removing the contaminants from sewage to produce liquid and solid (sludge) suitable for discharge to the environment or for reuse. We know that 75% of the world’s fresh water resources are contaminated, the remaining is fast disappearing. Waste water is produced from toilets, baths, showers, kitchens, sinks, and so forth that are disposed of via sewers.

Why Sewage Treatment Plant

One question that many people might ask is, “Why not simply dump this wastewater onto the ground outside the house, or into a nearby stream?” If you release wastewater directly into the environment, things get very smelly very fast. Human waste naturally contains bacteria that can cause disease. Once water becomes infected with these bacteria, it becomes a health hazard. The increased algae, reduced oxygen and murkiness destroy the ability of a stream or lake to support wildlife, and all of the fish, frogs and other life forms quickly die. That’s why communities build wastewater treatment plants and enforce laws against the release of raw sewage into the environment.

Stages in Sewage Treatment Plant

Pre-treatment: Pre-treatment removes materials that can be easily collected from the raw wastewater before they damage or clog the pumps and skimmers of primary treatment clarifiers. The influent sewage water is strained to remove all large objects carried in the sewage stream. This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants a manually cleaned screen may be used.

Primary treatment: Conventional sewage treatment may involve three stages, called primary, secondary and tertiary treatment. Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment.

Secondary treatment: Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment. Secondary treatment removes dissolved and suspended biological matter.

Tertiary treatment: Tertiary treatment is effected by sand filters, mechanical filtration or by passing the effluent through a constructed wetland such as a reed bed or grass plot. Tertiary treatment is sometimes defined as anything more than primary and secondary treatment. Treated water is sometimes disinfected chemically or physically (for example by lagoons and micro filtration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes.

Sludge Treatment

Sludge produced by sewage treatment is organic in nature and contain useful amounts of plant nutrients such as nitrogen, phosphorus and essential trace elements. The first objective should be to utilize the sludge as a fertilizer or soil conditioner on agricultural land. Sludge is treated using a variety of digestion techniques, the purpose of which is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting.

Anaerobic digestion generates biogas with a high proportion of methane that may be used to both heat the tank and run engines or microturbines for other on-site processes. In large treatment plants sufficient energy can be generated in this way to produce more electricity than the machines require. Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen.

Aerobic digestion is a bacterial process occurring in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, bacteria die and are used as food by other bacteria.

Composting is also an aerobic process that involves mixing the wastewater solids with sources of carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat.

Role of bacteria

In the typical sewage treatment plant, oxygen is added to improve the functioning of aerobic bacteria and to assist  them in maintaining superiority over the anaerobes.  Aerobic bacteria require oxygen for life support whereas anaerobes can sustain life without oxygen. Facultative bacteria have the capability of living either in the presence or in the absent of oxygen. Microorganisms and their enzyme systems are responsible for many different chemical reactions produced in the degradation of organic matter. As the bacteria metabolize, grow and divide they produce enzymes. These enzymes are high molecular weight proteins. All treatment plants should be designed to take advantage of the decomposition of organic materials by bacterial activity.

Sewage Treatment Plant is becoming ever more critical due to diminishing water resources, increasing waste-water disposal costs, and stricter discharge regulations that have lowered permissible contaminant levels in waste streams. Effective waste-water collection and treatment are of great importance from the standpoint of both environmental and public health. In recent years, there has been growing interest in waste-water reuse as a major component of water demand management.

Recommended Sewage Treatment Plants for Apartments

Sewage Treatment plant system can be opened or fully sealed. Some apartments adopt partially sealed system. One of main disadvantage of opened or partially sealed system is sewage is broken down by bacteria and during this process methane is released into campus or at ground floor level. The gas released from sewage chamber due to biological processing (by bacteria) will contain methane. Methane is highly combustible and toxic.

Sewage Treatment Plant with opened or partially closed is not recommended for apartment or densely populated areas. In this system smell can be sensed for miles. This system is only useful for large sewage treatments. For small residential units a completely sealed package unit is recommended. Sewage compartments with methane gas should be located away from car parking or area that can cause ignition or spark. Any ignition or spark can easily catch fire in sewage compartments with methane gas.

For apartments or for small residential units, automated system is recommended as this will reduce maintenance cost. Disadvantage of manually operated system is, there are chances of human error of dry running the pumps or not starting pumps in time. This can result in machinery breakdown or overflow of sludge. Automated system helps to prevent breakdown or over flow and association can save more than Rs 15000 per month on maintenance cost. Manually operating system is cheaper to install comparing to automated system but cost of maintenance is higher when comparing with automated system. The system release methane. This is supposed to be in sealed chambers and then released at a point few meters above the tallest point of the building. This should be taken care while designing sewage treatment plant for apartments or high-rise. In Metropolitan cities and urban areas, sewage treatment plant or mini sewage treatment plant is mandatory for getting a new water connection. Please contribute your knowledge on installing or setting up sewage treatment plant. If your apartment complex has sewage treatment plants, feel free to share your ideas in maintaining Sewage Treatment Plants.