Municipal Wastewater Treatment

Municipal wastewater is defined as wastewater from households or a mixture of wastewater from households and of industrial origin as well as precipitation water. The pollution of household wastewater is mainly due to the flushing of toilets, kitchen, and cleaning water polluted with bacteria, viruses, washing and cleaning agents including dirt and rests of food. Sewers collect sewage and wastewater from homes, businesses, and industries and deliver it to wastewater treatment facilities before it is discharged to water bodies or land, or reused. It is among the most important factors responsible for the general level of good health in each country.

 It is comparatively easy to almost completely break down these pollutants with the help of microorganisms used in wastewater treatment plants. Also, the nutrient concentration in wastewater may largely be reduced with the help of particular bacteria. The main purpose of the municipal wastewater treatment plant is to treat wastewater from homes. The plant does the necessary treatment processes to ensure the water is safe for direct discharge into the river.

The municipal wastewater treatment plant processes

The three main types of sewage treatment plants are Biological Filter, Pasveer Ditch, and Activated Sludge.

Biological filtration

This type of sewage treatment uses a huge cylindrical tank or a series of tanks that have plastic pieces or stones that are sprayed with the screened sewage via a mechanical rotating arm that is moving over the surface of the bed

Activated sludge

This municipal wastewater treatment style was invented in 1914 and has remained popular to date. In this method, a culture of bacteria is developed in a large tank as well as on the lanes that contain settled sewage. Oxygen, either as air or as pure oxygen, is then introduced mechanically into the sewage. This helps to improve the oxidation process

Pasveer Ditch

This is not very common in most regions of the world but it is very popular in mainland Europe. This system is made up of an oval-shaped channel that is 2-3 meters deep which is used to convey the sewage after it has undergone primary treatment. The sewage is then aerated and circulated in a ditch with the help of rotors that are mounted at different points all over the ditch. How fast the rotation goes will be determined by the oxygen demand of the sewage.

The sewage treatment plants typically adhere to four stages of treatment namely;

1. Primary treatment
2. Secondary treatment
3. Tertiary treatment
4. Sludge treatment.

Primary treatment in the wastewater treatment plant

The first step in a wastewater treatment process utilizes primary or mechanical systems to remove large suspended and floating solids from raw sewage to avoid blockage and damage of valves, pumps, channels, orifices and other components of the system. The wastewater is first of all taken into a settling tank, also referred to as a Grit Removal unit, after which it goes through screening in order to remove large organic material. It starts with coarse screens made of bars with a spacing of 6mm and then followed with finer screeners and drum filters. Screening is sometimes combined with maceration, which is the shredding of raw sewage and then followed by a process of crushing the solids into tiny particles. The screened sewage is then passed into a sedimentation tank for settling most of the suspended matter. Dissolved and colloidal solids are however not removed because they need further treatment which usually happens at the secondary stage.

Secondary treatment or biological wastewater treatment

Secondary wastewater treatment is a biological treatment process designed to reduce the amount of organic materials in wastewater. high concentrations of naturally occurring bacteria are confined in treatment tankage and combined with protozoa and other microbes to form activated sludge. The activated sludge bacteria break down organic molecules into inert substances creating effluent water suitable for discharge to rivers, lakes or streams. Secondary treatment involves the removal of biodegradable organic matter (BOD) and suspended solids (TSS) through the processes of aeration and filtration. Secondary treatment is typically characterized as producing a treated wastewater effluent with a BOD of 25 mg/L or less and a TSS of 30 mg/L or less. 

Inorganic waste can to a small degree be treated biologically but for the most part, it will need chemical treatment. If the secondary stage is done properly, the outcome will be treated sewage that has very little toxicity. The growth of the microbial population is facilitated by the nutrients that are provided by the raw sewage. But in addition to the nutrients, the right temperature, pH as well as dissolved oxygen must also be in place in order for the environment to be optimum for the growth of bacteria. On average, the most ideal environment should be approximately 25-32 degrees Celsius, 5.5-9.5pH and 2mgl of DO.

Secondary Wastewater treatment is divided into two different treatment processes:

1. Aerobic Treatment: Aerobic wastewater treatment is a biological treatment that uses oxygen to break down organic matter and remove other pollutants like nitrogen and phosphorus. Aerobic treatment technologies are:

• Activated Sludge Process (ASP) / Extended Aeration System (EAS)
• Sequential Batch Reactor (SBR)
• Moving Bed Bio Film Reactor (MBBR)
• Membrane Bioreactor (MBR)

2. Anaerobic Treatment: Anaerobic treatment is a process where wastewater or material is broken down by microorganisms without the aid of dissolved oxygen. However, anaerobic bacteria can and will use oxygen that is found in the oxides introduced into the system or they can obtain it from organic material within the wastewater. Anaerobic treatment technology is Up-flow Anaerobic Sludge Blanket Reactor (UASB)

 Secondary treatment can also include disinfection to remove harmful microorganisms using chlorine or ultraviolet light.

Tertiary treatment in the wastewater treatment plant

The most advanced wastewater treatment process removes nutrients, suspended solids, and organic materials from wastewater so it is able to be discharged safely into the environment.

Tertiary treatment involves the reduction of nutrients (nitrogen, phosphorus) through more advanced treatment processes When needed, it sometimes involves using alum to remove phosphorus particles from the water. Alum also causes any solids that were not removed by primary and secondary wastewater treatment to the group so they can be removed by filters. When necessary, the filters are backwashed to remove the build-up of floc, which allows the filters to continue operating effectively.

Most wastewater treatment plants use at least one tertiary filtration system to accomplish coagulation-assisted filtration, absolute barrier filtration, or nutrient, a removal which includes biological denitrification and chemical phosphorus removal. As wastewater is purified to higher degrees by such treatment processes, the treated effluent can then be reused for irrigation, recreational use, or water reuse

DISINFECTION

An important part of wastewater treatment involves the addition of chlorine to the final effluent before discharge. This process injects chlorine into the headworks of a serpentine effluent detention chamber. Chlorination in wastewater treatment kills bacteria and viruses and eliminates parasites such as Giardia and Cryptosporidium, which can cause very serious illnesses. In summary, this process disinfects water so that it is safe to reuse or recycle.

DICHLORINATION

The final stage of the tertiary wastewater treatment process involves removing the chlorine that was used to disinfect the water. This step is very important because chlorine is harmful to aquatic life. Chlorine also reduces biological water quality when it is present in high concentrations.

To remove the chlorine, a compound called sodium bisulfite is added to the water. Chlorine ions in the water react with this chemical and are removed. Once the chlorine concentration has been reduced to a safe level, the treated water is now considered clean enough to be safely released into the environment.