Equalization and Homogenization
Category / type: industrial (or civil) treatment.
Application: small, medium, large and municipal wastewater treatment.
Size: from 3 up to 40 m3 single line.
Flow: from 1,0 up to 100 m3 per day.
Normative references and guarantees: EN 12050-2:2001.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
Chance: system revamping.
Flow equalization system mitigates changes in flow rate by providing storage that hold exceeding water (when inlet flow increase) or supply additional water (when inlet flow decrease under design parameters). The quantity of wastewater might fluctuate dramatically during the day depending on working cycle, therefore it is convenient to equalize the flow before feeds it to the various treatment steps.
Flow equalization is one of the most effective management procedures for wastewater treatment plants. Uniform flow condition in fact improves efficiency. Variations in flow composition could be due to many reasons, including the cyclic nature of industrial processes, the sudden occurrence of storm water events and seasonal variations. Flow rate can be null in some part of the day.
Large plants normally require an equalization phase, even thought flow peaks represent problems for small ones also.
Equalization tanks are used to limit the effects of sudden and unexpected spikes in pollution load (shock loads).
The system must have some logical control (electric panel and level controls) to automate pumps that feed wastewater in the plant, to keep the plant operating at nearly constant conditions
There are two kinds of equalization method: in-line or side-line. In-line is the most common solution, the second is more suitable for combined sewer system for both wastewater and storm water (when there are heavy rains). Side line methods does not get to complete homogenisation but saves energy.
How It Works
The equalization tank(s) must be large enough to absorb and dampen the pick flow or possible spills during the process. The engineering issue is deciding how large an equalization basin is required to allow the treatment processes to operate with a steady, average flow.
The first method relies on computing the equalization volume, based on the excess daily average flow storage. The required volume is also graphically determined by constructing a hydrograph (Rippl Diagram). The function of the basin is to store exceeding daily flow and divert it during times, when the inflow is less that the average daily flow.
The second method computes the volume based on mass loading variations.
The aeration in the equalization vessel can reduce the BOD from 10 up to 20 %. To simplify the analysis, it is assumed that it is thoroughly mixed with no chemical or biological reaction.
For a small domestic wastewater flow or medium-sized plants, the minimum volume (W) is calculate as:
16 [hour] discharge time;
Q16 [m3 per hour] medium flow on discharge time;
Q24 [m3 per hour] daily medium flow rate.
For small treatment plants, where daily flow pattern is not known, a very simple approach is possible. The minimum equalization volume is equal from 20% up to 40% of the daily volume.
The smaller the plant, the higher percentage is normally recommended.
An equalization chamber is always provided by electric pumps; two pumps are normally installed as back-up solution and for maintenance. Pumps usually work with constant flow controlled by a normal on/off timer or with a variable flow (inverter motor) controlled by a flow meter, fairly expensive solution. The system must have a logical control to decide which pumps are on and to add back the surplus wastewater at rates that keep the rest of the plant operating at nearly constant conditions
Placement of an equalization tank following primary treatment minimizes operation and maintenance, and minimizes requirements for solids removal, aeration, and odour control equipment.
Pretreatments are able to manage variable pollutant rates and they also protect pumps and mechanical parts from sand and solids.
Mixers are often installed in equalization basins to achieve homogeneity and to aerate wastewater preventing septic condition. In small plants solution, aerators and mixers can be the same system (Venturi system).
Operation and maintenance
Equalization chamber requires periodical maintenance to check pumps and mixers and all electrical or mechanical components.
Maintenance personnel shall be able to access the tank safely. Skilled personnel from authorized company is needed.
Aeration and mixing can cause odour problems, therefore a good vents system must be provided.
A part of recirculated sludge from secondary sedimentation, can be sent upstream to equalization tank. This operation gets a good pre-oxygenation and reduces odours.
EN 12050-2:2001: Wastewater lifting plants for buildings and sites. Principles of construction and testing. Lifting plants for faecal-free wastewater.
Features and benefits
Flow equalization is a method used to overcome operational problems and flow rate variations, to improve the performance of downstream processes and to reduce size and cost of downstream treatment facilities. Flow equalisation is often used to prevent large flow rate fluctuation, temperature, and contaminant concentrations.
An equalization plant is always required in industrial wastewater treatment plants, where cycling activities produce discontinuous flow.
Equalization chamber provides increasing benefits with increasing plant complexity.
A constant and uniform flow condition guarantees wastewater treatment plant efficiency. It results in more control of various physical, chemical and biological treatment processes and more reliability. Costs are reduced thank to the elimination of excessive peak treatment volume and the decrease of peaking condition operation period.
The supernatant from treated sludge or other treatment as filters backwash, rich in organic matter, usually should put it back (primary phases) for more treatment; the equalization step is a good place to add back this water.
A constant flow rate is a great advantage for biological reaction but also for chemicals dosage as pH control, chlorination, nutrients, chemical phosphorus removal, etc.
- sizing plants on medium flow and loads;
- shock loads management (planned and unplanned);
- biological reactor protection;
- reagent dosing best management;
- pre-aeration (for septic waste);
- exact flow distribution on different lines.
- electric consumption (pumps);
- odours (aeration);
- regular maintenance.