Screens and balancing form the first stage of any industrial effluent treatment scheme, and it addresses two different tasks. The screen mechanically retains coarse matter and fibre that would otherwise disable the pumps and mechanisms of the stages downstream. The balancing tank evens out flow and composition over time, turning a slug discharge into the steady feed for which everything that follows is designed.
Principle
The screen is set across the flow in the inlet channel or in the receiving chamber and retains everything coarser than the aperture — the spacing between the bars or the mesh size of the face. The velocity through the aperture at the maximum design flow is kept at no more than about 0.9 m/s: any faster and soft matter is forced through the face, too slow and the required cross-section grows. The velocity in the inlet channel, by contrast, is kept at no less than about 0.4 m/s, otherwise sand settles ahead of the screen and the channel needs cleaning more often than the screen itself.
As screenings accumulate the face blinds over, the head loss rises and the level upstream of the screen goes up — that difference in levels is the sign that the face needs cleaning. A manual screen is raked on a schedule; a mechanical one is cleaned by rakes or brushes on a timer or on the level difference, discharging the screenings onto a conveyor or into a container. An aperture finer than about 5 mm cannot be maintained by hand under continuous flow, so choosing a fine aperture is at the same time choosing mechanical cleaning and an organised route for the screenings.
The aperture is selected from what stands behind the screen rather than by the rule that finer is better. Coarse screens are conventionally taken at apertures of some 15–50 mm, medium screens at 5–15 mm and fine sieves at 0.5–5 mm; the boundaries are conventional and are drawn differently in different sources. A submersible pump calls for an aperture matched to the free passage of its impeller; the scraper mechanism of a flotation unit and the space between the plates of a lamella clarifier call for a markedly finer sieve. Every step towards a finer aperture increases the quantity of screenings and the demands on their handling.
The balancing tank is a vessel that receives effluent as production delivers it and discharges it at a constant rate. The working volume is not estimated by eye but derived from the daily discharge profile: cumulative inflow is plotted hour by hour, the uniform draw-off is subtracted from it, and the largest difference is the volume required. To that are added the dead volume that covers pump suction and mixer operation and the freeboard up to the emergency level. For shift work with a washdown at the end of each shift the calculation usually gives a few hours of average flow, but that figure cannot be substituted for the profile: two plants with the same daily volume can need tanks that differ several-fold.
Mixing and pumped feed are parts of the stage, not accessories to it. Mixing by stirrer or by air keeps solids in suspension and blends portions that arrived at different times; the specific power of mechanical mixing is of the order of 4–8 W per cubic metre of volume at a moderate suspended solids content, and aeration is of the order of 0.01–0.015 m³ of air per cubic metre of volume per minute. The pump on the outlet delivers the design flow regardless of the level in the tank; it is the pump, not the vessel, that makes balancing what it is, and its delivery is therefore regulated rather than switched on and off by level.
Principle
A balancing tank evens out not only flow but also concentration and pH, and on the chemical side that shows more plainly than on the hydraulic one. Acidic and alkaline slugs arriving in the same vessel a few hours apart partly cancel each other out: part of the neutralisation is done without a single gram of reagent. Without balancing the same reagent is dosed against a rapidly moving set point and with a large margin, because the controller always lags behind the front of the slug. The difference shows up not in the design but in the annual reagent consumption and in the steadiness of the figures at the outfall.
A vessel without mixing is not a balancing tank but a settling tank, and a poor one. The heavy fraction lies on the floor and compacts, the light fraction rises as a crust, and between them runs a short-circuiting flow that leaves with much the same uneven composition it arrived with. At the same time the sludge accumulated on the floor turns to anaerobic breakdown within a few hours at an effluent temperature of 20–30 °C, and hydrogen sulphide appears at the outlet — an odour that was not in the raw effluent at all. Such a vessel does not merely fail to solve the task; it creates a new one.
An error in the volume of the balancing tank costs more than an error in any other part of the scheme, and it costs in both directions. Too small a volume does not damp the slug: the peak passes straight through, the reagent side and the next stage receive flow and concentration above their design values, and the scheme goes out of control at precisely the moment of maximum load. Too large a volume with insufficient mixing and no aeration means many hours of residence for effluent rich in organics, that is, anaerobic fermentation, odour and sulphide corrosion of concrete and metal. Both errors can be put right only by construction work.
Practice
The stage improves no single parameter in a spot sample, and that regularly raises questions at handover. The mean concentration leaving the balancing tank is close to the mean entering it: what changes is not the mean but the spread — the peaks disappear, the peaks the whole downstream scheme would otherwise have to be sized for. Judging a balancing tank by one sample is pointless; a series of samples across a shift together with a flow record is what is needed.
This stage determines the size of the rest of the scheme. The structures downstream are sized for the flow the balancing pump delivers, not for the peak of a washdown, and where the pattern is markedly uneven those two figures differ by a factor rather than a percentage. A tank with a mixer and a pump usually costs less than a flotation unit, a dosing station and a clarifier all sized for the peak.
Screenings are a waste stream in their own right, not an incidental nuisance. They are wet, they carry organics, they go septic quickly and they call for a closed container, a place for it, a set collection interval and a receiving contractor. This has to be planned together with the choice of aperture: the finer the aperture, the more screenings there are.
We do not manufacture the screen, the tank, the mixer or the pumps. Our part of the work is to take the discharge profile, calculate the working volume and the delivery from it, choose the aperture and the type of screen from the effluent and from the requirements of the next stage, assemble the pipework and the controls, supply the equipment, commission it and service it.
Limits
Practice
The cases are typical examples, not site reports.
Scheme
This stage never stands on its own: it always sits ahead of something and is set up from the requirements of what follows. The order of the units within it is not reversible either — screen first, balancing tank second: coarse matter and fibre must reach neither the mixer nor the feed pump.
Where the effluent carries sand and a heavy mineral fraction, a grit trap is placed between the screen and the balancing tank. Without it the mineral part builds up on the floor of the tank, reducing the working volume and forcing unplanned cleaning; mixing does not hold it fully in suspension.
The outlet of the balancing tank is the reference point for the whole scheme downstream. The flow meter belongs here: reagent doses are calculated from it, and the flotation unit, the clarifier and the biological stage are selected from it. The design delivery of this pump is the flow everything downstream is designed for.
A bypass line or an emergency channel is provided at the screen for maintenance and for a blinded face, and freeboard up to the emergency level with an overflow to a predetermined point is provided at the tank. Both belong to the safety of the site rather than to the quality of treatment, and that is exactly why they are the ones most often left out.
Scheme
Operation
Energy
Specific energy consumption is made up of mixing and of lifting the water, and comes to some 0.05–0.15 kWh per cubic metre of effluent. The first term follows from the specific mixing power and the residence time: at 4–8 W per cubic metre of volume and a residence time of a few hours it is a few hundredths of a kilowatt hour per cubic metre. The second follows from the head: a lift of 5–10 m at a pump set efficiency of about 0.6 gives roughly as much again. An aerated balancing tank consumes appreciably more, since the blower runs continuously.
Consumables
What goes to drain
The stage produces two waste streams. Screenings are a wet mass with a high organic content; their quantity depends on the aperture and on the character of the effluent, they are collected in a closed container and transferred to a licensed contractor, and the hazard class and the permitted route are determined by their composition, not by the type of equipment. The second stream is the sludge on the floor of the balancing tank: even with mixing in operation part of the heavy fraction accumulates, so emptying the tank, access for cleaning it out and the interval for that operation are part of the design. Dissolved matter the stage does not remove at all and passes on down the scheme with its mass unchanged.
Sizing
What we account for
What a wrong choice costs
Sizing
Questions
A margin on capacity does not replace balancing, because the two address different things. A stage sized for the peak flow works at that flow for a few minutes a day and spends the rest of the time well below its design flow, which disturbs the hydraulic regime of a flotation unit and of a clarifier alike. Beyond that, a margin on flow does not even out composition: concentration and acidity peaks pass straight through whatever the size of the structure.
Yes, otherwise it is not a balancing tank. Without mixing the heavy fraction lies on the floor, the light fraction rises as a crust, and a short-circuiting flow passes between them keeping its uneven composition. The accumulated sludge turns to anaerobic breakdown within a few hours at the usual temperature of industrial effluent, and hydrogen sulphide appears at the outlet. Mixing is done by a mechanical stirrer or by air; the second method also slows the onset of septicity.
The volume is calculated from the daily discharge profile: cumulative inflow is plotted hour by hour, the uniform draw-off is subtracted from it, and the largest difference gives the working volume required, to which the dead volume and the freeboard up to the emergency level are added. For shift work with a washdown at the end of the shift the result usually comes to a few hours of average flow, but that guide is checked by calculation: at the same daily volume the tank required differs several-fold from one plant to another.
The functions differ, although the two are sometimes combined in one vessel. A receiving chamber collects the effluent and lifts it from the level of the gravity drain, and its volume follows from the permissible starting frequency of the pump. A balancing tank evens out flow and composition, and its volume follows from the discharge profile. Where the two are combined the volume is taken from the second task, and mixing and regulated delivery remain mandatory.
Odour comes not from the stage itself but from a regime in which effluent stands without oxygen: hydrogen sulphide then forms that was not in the raw effluent. The first measure is mixing and, at a high organic load or an elevated temperature, aeration. Beyond that the tank is covered and the extract is taken to a safe point or to air treatment, and a closed container is provided for the screenings.
Tell us about your site — our engineer will prepare a process design, a specification and a quotation within one business day.