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Sewage Water Treatment Plant Manufacturers

A Brief Description

In the coming years, water is going to be the most debated environmental challenges globally. The global demand for fresh water is increasing rapidly. Water for industrial and residential usage is burgeoning with no signs of abatement.

Due to increasing water scarcity, countries developed technologies to desalinate sea water to compensate for the shortage of fresh water for human consumption. Despite this, today global water scarcity is looming large and worldwide there is an attempt to conserve water. This has resulted in nations looking for ways to reduce the water consumption and also recycle and reuse the waste water. The concept of sewage treatment and industrial effluent treatment was born out of this necessity. The reuse of treated waste water has today become an ecological imperative.

Treatment and recycling of waste water (sewage, sullage, industrial effluent) has been acknowledged as an economical and commercially viable proposition. Recycling of waste water will drastically reduce the rate of depletion of surface and ground water.

As water demand and environmental needs grow, waste water recycling will play a vital role in our overall water supply. Water recycling along with water conservation will help us sustain and manage our vital water resources.

Waste water treatment is the process of removing physical, chemical and biological contaminants from the waste water. These plants operate on the principle of degradation of organic matter achieved through bacterial activity. This bacterial activity can be facilitated through biological processes like aerobic treatment (bacterial growth in oxygenated environment) and anaerobic treatment (bacterial growth in the absence of oxygen).

In the aerobic process, oxygen is generally supplied in diluted form (in the form of air) to oxygenate water. The process of supplying air may be different viz. through fixed or floating aerators or through diffused aeration. This facilitates rapid bacterial growth which eats away the organic matter and cleans up water. As a result, a majority of the biological contaminants found in the waste water is removed.

In the anaerobic process, bacterial digestion is carried out in the absence of oxygen. This process can either be thermophilic in which sludge is fermented in tanks at a temperature of 55 ˚ C or mesophilic at a temperature of around 36 ˚ C.


Technology and Process Description

Conventional waste water treatment is mostly aerobic and is a multistage process. The typical stages adopted in waste water treatment can be classified as follows :

i.Primary Treatment ii. Secondary Treatment iii. Tertiary Treatment

Primary Treatment

Primary treatment reduces oil, grease, fat, sand, grit, and coarse/ large solids in the waste water. This step is done entirely with machinery and hence is also known as Mechanical Treatment.

In this process stage, the raw sewage (influent) is pumped into the inlet chamber of the treatment plant where the bar screen is located. All the small and large suspended / floating particles of various sizes such as twigs, leaves, rags, sticks, condoms, sanitary napkins, gunny bags, tampons, vegetable / fruit peels, other debris are removed by using a manual or automated mechanically raked bar (coarse or fine) screen. This water is further sent to a grit chamber where the velocity of the incoming waste water is carefully controlled to allow sand, mud, ash, egg shells, stones and other minute inert materials to settle but the suspended organic material remains in the water due to the velocity factor. The early removal of these particles avoids damage to the pumps and other equipment used in the subsequent treatment stages. A grit classifier followed by a conveyor to transport the grit to a separate container for disposal is an optional equipment. The accumulated grit is generally used for land filling. In case of excess oil, grease and fat, manual or mechanized oil skimmers / oil separator systems for removal of floating oil and grease/fat will be provided.

This water goes into an equalization/neutralization tank to achieve homogeneity. The main purpose of the primary stage is to equalize the flow and produce a generally homogeneous liquid capable of being treated biologically. Coarse diffusers connected to the twin lobe air blowers are used to homogenize the mixed liquor. A level controller for auto-transfer of this mixed liquor to the aeration tank is optional. This water is further sent for secondary treatment at a consistent flow rate.


Secondary Treatment

Waste water from the primary treatment stage flows into the aeration tank where the waste organic matter is aerated. Secondary treatment is designed to significantly degrade the biological content of the waste water derived from human waste, food waste, soaps and detergent. This results in the reduction of organic load of the waste water. ie., Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). The majority of the domestic, municipal and industrial plants use aerobic processes to treat the mixed liquor. In this process, the waste water is oxygenated by mixing atmospheric air to multiply the bacteria and protozoa which start consuming the biodegradable and soluble organic contaminants and bind much of the less soluble fractions into floc particles. This treatment is also known as Biological Treatment. In case of a dip in MLSS Activated Bacterial Growth is achieved / accelerated with the help of cow dung or urea.

Oxygenation / aeration can be achieved by using different types of aerators like fine-pored diffusers, fixed aerators, floating aerators, spray aerators and air grids. There are various methodologies adopted in secondary treatment. ie., fixed media / fluidized bed and suspended growth.

In the first system, biomass grows on media and the waste water passes over its surface. In the second system, the biomass is well mixed with the waste water. Typically, fixed media systems require smaller footprints than for an equivalent size of suspended growth system.

The aerated waste water from the aeration tank flows to secondary settling / clarifier tank where separation of biological sludge takes place. The clarifier tank is also known as sedimentation tank. This clarifier tank can be circular or rectangular with bottom slope equipped with a feed well drum, sludge chamber at the bottom, sludge scrapper and overflow weir. Settling tanks operate on the principle of slowing the flow sufficiently so that slow settling materials can sink and be collected and removed. The supernatant flows into the next treatment stage.

The efficient separation of the sludge is ensured by using appropriate clarifier mechanisms. Various types of clarifier mechanisms are provided to separate the sludge and water. ie., lamella clarifiers, plate separators, tube settlers etc.

MORF INDIA offers a variety of effluent / application-specific biological treatment processes like ASP, FBBR, SAFF and MBR for this purpose.


Sludge (biomass) Recirculation

The secondary stage aeration process creates biomass (activated sludge). A portion of this sludge is re-circulated back to the aeration system by using a centrifugal pump to maintain adequate MLSS (reseeding the fresh sewage) levels in the aeration tank. Excess sludge is removed from the treatment process to keep the ratio of biomass to food supplied (waste water) in balance (F:M ratio)

The sludge recirculation pumps are designed / chosen to transfer sludge without deterioration of flocs. Clarifier mechanisms with specific retention time and rise velocities are used to handle the bio-sludge. This system eliminates sludge bulking and also ensures uniform and consistent sludge is re-circulated. The excess sludge, also known as surplus activated sludge, may further be treated by aerobic or anaerobic processes before disposal.


Tertiary Treatment

Tertiary treatment is the final stage of treatment which raises the effluent quality to the required standards before it is discharged to the receiving environment. The objective of the tertiary treatment is to filter the residual particles (suspended solids, turbidity, residual organics, colour, odour, colloidal matter) and disinfect the water after it has undergone primary and secondary treatment stages. Multiple tertiary treatment processes may be used depending upon the application (end-use) of the treated water. This treatment is also known as Physical-Chemical Treatment.

The standard tertiary treatment comprises disinfection (elimination of pathogens), sand filtration for removal of suspended particles, turbidity and activated carbon filtration to remove residual chlorine, colour, odour and adsorb organics. Ultra-filtration modules and RO plants can also be given to fine polish / purify the effluent for other advanced and critical uses.


Nutrient Removal

Waste water may also contain high levels of nutrients (nitrogen and phosphorous) that in certain forms may be toxic to fish and invertebrates at low concentrations (e.g. ammonia) or can create nuisance conditions in the receiving environment (eg. weed or algal growth). Algae can produce toxins and their death and consumption by bacteria (decay) can considerably deplete oxygen levels in the water. The removal of nitrogen and phosphorous from waste water can be achieved either biologically or by chemical precipitation.

In the biological process, nitrogen is reduced from ammonia to nitrate (nitrification) and then from nitrate to nitrogen gas (denitrification) which is then released to the atmosphere. These form conversions occur in carefully controlled conditions to encourage appropriate biological process to happen. Sand filters, lagooning and reed beds can all be used to reduce nitrogen. Phosphorous removal can be effected biologically by a process called enhanced biological phosphorous removal. In this process, specific bacteria called polyphosphate-accumulating organisms are selectively enriched and they accumulate large quantities of phosphorous within their cells. When the biomass enriched in these bacteria is separated from the treated water, the bacterial biosolids have a high fertilizer value. Phosporous removal can also be achieved by chemical precipitation with ferric chloride or aluminium sulphate. The resulting chemical sludge, however, is difficult to dispose of and the use of chemicals in the treatment process is expensive and makes the operation relatively difficult.


Disinfection

The purpose of disinfection in the treatment of waste water is to eliminate the number of living microorganisms in the water so that the water can be discharged back into the environment. Common methods of disinfection include ozone, chlorine, or UV light.

Chlorination remains the most common form of waste water disinfection due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically de-chlorinated.

Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3 . Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with and thereby destroys many disease-causing microorganisms. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for highly skilled operators.


Sludge Disposal

The processed sludge containing coarse and fine bio-solid particles generated as a by-product of the primary, secondary and tertiary treatments of sewage must be treated and disposed of in a safe and effective manner. Sludge handling is aimed at increasing the sludge concentration to reduce the load to the downstream treatment equipment. Generally, sludge is thickened (dewatered) to reduce the volume of the sludge for disposal. The choice of waste water solid (sludge) treatment method depends on the amount of solids generated and other site-specific conditions.

Processes for reducing water content include lagooning in sludge drying beds to produce dry cakes. Filter presses can be used to mechanically filter the sludge through cloth screens to produce solid sludge cakes. Sludge is also thickened by using centrifuges which separate the solid and liquid. Dry sludge cakes are used as a natural fertilizer. Alternatively, this sludge can also be used for landfill. Composting is also an option for the disposal of sludge.