Effluent treatment plant design is a complex and multidisciplinary work that requires knowledge and skills in various fields such as water chemistry, biology, hydraulics, engineering, civil. Purpose of Effluent treatment plant design is to achieve the best possible treatment results with the lowest possible costs and environmental impacts. Effluent treatment plant design can be done for specific individual industries or for common effluent treatment plants that treat multiple industries. Effluent treatment plant design can also include plant operation and maintenance services, plant reports, layout drawings.
What is an effluent treatment plant?
An effluent treatment plant (ETP) is a process that treats hazardous industrial wastewater before discharging it to the environment or reusing it within process. The main objectives of an ETP are to protect the environment from pollution, conserve water, comply with regulatory standards, and reduce operating costs, but do remember that ETPs are not cheap.
How to design an effluent treatment plant?
The design of an ETP depends on various factors, such as:
- The quality and quantity of the wastewater generated by the industry ie Volume of the Effluent
- The land availability, location, and site characteristics
- The required results parameters of the treated water
- The budget and operational constraints
The design process involves selecting and designing physical, chemical, and biological treatment processes that can remove contaminants such as suspended solids, organic matter, nutrients, metals, pathogens, Viruses, smell/odour, colour from the wastewater. Designing process also involves hydraulic design methodologies that can ensure required flow rates, retention times, mixing, aeration, MLSS, F/M ratio in the treatment units.
Some of the common treatment processes used in ETPs are:
- Preliminary treatment: This includes screening of non-biodegradable items , grit removal, equalization for making homogenous mixture, neutralization by pH adjustment.
- Primary treatment: This includes sedimentation, flotation, filtration, Coagulation & Flocculation to remove settleable and floatable solids from the wastewater if required by the action of chemical precipitation.
- Secondary treatment: This includes activated sludge, trickling filter, rotating biological contactor, anaerobic digestion, MBBR, SBR, SAFF etc. to remove biodegradable organic matter and nutrients from the wastewater using specifically selected microorganisms which can withstand temperature shocks.
- Tertiary treatment: This includes disinfection by UV, Ozone, Chlorine, sand filtration, membrane filtration, reverse osmosis, ultra filtration, nano filtration. to remove residual contaminants and pathogens from the wastewater and enhances its quality for reuse, storage or discharge.
The design of an ETP should also consider the following aspects:
- Plant layout
- Plant operation and maintenance
- Plant safety and reliability
- Plant monitoring and control
- Plant performance evaluation and optimization
An example of effluent treatment plant design
To illustrate the design of an ETP, let us consider a hypothetical example of a textile industry that generates 1000 m3/d of wastewater with the following characteristics:
| Parameter | Unit | Value |
|-----------|------|-------|
| pH | - | 9 |
| BOD | mg/L | 500 |
| COD | mg/L | 1000 |
| TSS | mg/L | 500 |
| TDS | mg/L | 2000 |
| NH3-N | mg/L | 50 |
| NO3-N | mg/L | 10 |
| PO4-P | mg/L | 5 |
The industry has a land area of 5000 m2 available for the ETP and wants to reuse the treated water for irrigation purposes. The required parameters of the treated water are:
| Parameter | Unit | Value |
|-----------|------|-------|
| pH | - | 6.5-8.5 |
| BOD | mg/L | <20 |
| COD | mg/L | <150 |
| TSS | mg/L | <10 |
| TDS | mg/L | <500 |
| NH3-N | mg/L | <5 |
| NO3-N | mg/L | <10 |
| PO4-P | mg/L | <1 |
The ETP design is based on the following assumptions and criteria:
- The wastewater flow rate is constant at 1000 m3/d
- The wastewater temperature is 25 °C
- The wastewater characteristics are as given in the table above
- The design life of the ETP is 20 years
- The design standards and codes are as per local regulations
The ETP design is subject to verification and validation by field tests, pilot studies, and simulations. The ETP design may also be modified or optimized according to the actual performance and feedback from the ETP’s end user.
The ETP consists of the following units:
Bar Screening: By a mechanical device that removes large solids such as rags, fibers, plastics, etc which are non-biodegradable, from the wastewater. The screen has a spacing of 2 inches.
Grit chamber: A tank that removes grit such as sand, gravel, stones, etc. from the wastewater by gravity settling. The grit chamber has a length of 10 m, a width of 2 m, a depth of 1 m, and a detention time of 1 min.
Equalization tank: A tank that stores and mixes the wastewater to make it homogenous and even out the variations in flow and quality. The equalization tank has a volume of 250 m3 and a detention time of 6 hours.
Neutralization tank: A tank that adjusts the pH of the wastewater by adding acid or alkali solutions. The neutralization tank has a volume of 50 m3 and a detention time of 30 min. The pH is controlled by a pH sensor and a controller that activates the chemical pumps. The target pH is 8.0.
Coagulation-flocculation tank: A tank that adds coagulants such as Alum and flocculants such a polyelectrolyte to the wastewater to precipitate the wastewater and removal of suspended solids, organic matter, and dissolved pollutants along with color. The coagulation-flocculation tank has a volume of 100 m3 and a detention time of 60 min. The dosages are determined by jar tests.
Primary clarifier/ Circular: A Circular Clarifier with bridge that separates the coagulated and flocculated solids from the wastewater by gravity settling. The primary clarifier has a diameter of 10 m, a depth of 3 m, a surface loading rate of 10 m3/m2.h, and a sludge removal rate of 0.5 m/h. The sludge is collected at the bottom and pumped to a sludge thickener. The effluent is collected at the top and sent to the secondary treatment. Clarifier can be Rectangular, square or circular in nature.
Activated sludge reactor: A tank that aerates and mixes the wastewater with microorganisms that consume the organic matter. The activated sludge reactor has a volume of 500 m3 and a detention time of 12 hours. The dissolved oxygen level is maintained at 2 – 2.5 mg/L by an oxygen sensor and a blower. The mixed liquor suspended solids (MLSS) concentration is maintained at 3600 mg/L by wasting excess sludge to the sludge thickener. The effluent is sent to the secondary clarifier. Here activated sludge reactor used is conventional type only without any media support for microorganisms.
Secondary clarifier: A tank that separates the activated sludge from the wastewater by gravity settling. The secondary clarifier has a diameter of 15 m, a depth of 4 m, a surface loading rate of 8 m3/m2.h, and a sludge removal rate of 0.5 m/h. The sludge is recycled back to the activated sludge reactor at a rate of 300 m3/h. The effluent is collected at the top and sent to the tertiary treatment.
Pressure Sand filter: A filter that removes residual suspended solids, organic matter from the wastewater by passing it through a bed of different size of sand & pebbles via pressure pump. The Pressure sand filter is backwashed periodically with air and water to remove the accumulated solids. The effluent is sent to the Activated Carbon Filter
Activated Carbon Filter: A Filter that polishes the treated water, removes color, adsorbs harmful bacteria & viruses.
Disinfection unit: A unit that kills or inactivates any remaining pathogens in the wastewater by applying chlorine or ultraviolet (UV) light or Ozone. The disinfection unit consists of either a chlorine contact tank or a UV reactor or Ozonation. The chlorine contact tank has a volume of 50 m3 and a detention time of 30 min. The chlorine dosage is adjusted by a residual chlorine sensor and a controller that activates the chlorine pump. The target residual chlorine level is 0.5 mg/L. The UV disinfection is done in line. The UV lamps are cleaned periodically to maintain their efficiency. The effluent is sent to the storage tank or irrigation system.
The ETP/Wastewater Treatment design also includes:
- Sludge thickener: A tank that concentrates the primary and secondary sludge by gravity settling. The sludge thickener has a diameter of 5 m, a depth of 3 m, a surface loading rate of 2 m3/m2.h, and a sludge removal rate of 0.1 m/h. The thickened sludge has a solids concentration of about 5% and is pumped to a sludge dewatering unit, Sludge thickener is constantly stirred by any mixer or coarse bubbles.
Sludge dewatering unit: A unit that reduces the moisture content of the sludge by applying pressure or heat. The sludge dewatering unit consists of either a belt filter press or a rotary dryer. The belt filter press squeezes the sludge between Plates and produces a cake with about 20% solids content. The rotary dryer heats the sludge in a rotating drum and produces a granular product with about 80% solids content. The dewatered sludge is disposed of in a landfill or used as a fertilizer or fuel.
Storage tank: A tank that stores the treated water for reuse or discharge. The storage tank has a volume of 500 m3 and a level sensor that controls the inflow and outflow of water. The storage tank also has an overflow pipe that discharges excess water to a nearby stream or drain.
Irrigation system: A system that distributes the treated water to the fields or gardens where it is used for watering plants.
The ETP design also includes:
Electrical and instrumentation system: A system that provides power and command control to the ETP reactors and devices. The electrical and instrumentation system consists of switchboards, cables, motors, sensors, controllers, indicators, alarms, pumps. The electrical and instrumentation system is designed to ensure safety, reliability, and efficiency of the ETP operation.
Civil and structural works: A works that provide support and protection to the ETP units and devices. The civil and structural works consist of foundations, walls, roofs, floors, platforms, stairs, railings, fences, etc. The civil and structural works are designed to withstand loads, vibrations, weather conditions like rain.
Richa Enviromental is active in the field of Effluent Treatment Plant since 2010