Wastewater discharge: what is monitored, the limits and the stages

A plant most often learns about discharge requirements not from documents but from the first fine or the first refusal to accept its effluent. By that point the workshop is already built, the sewer is connected, and the question sounds the same every time: what exactly was measured, and why has it become a problem. Here is what is monitored in wastewater, who sets the limits for it and what equipment brings those parameters down.

Who sets the limits

There is no single table of permissible values that fits any site. The limits are set by whoever receives the effluent: the water utility if the wastewater goes into the municipal network, the local regulator if the discharge goes into a water body or onto the ground. The conditions are recorded in the discharge permit or in the wastewater acceptance contract, and that is the only document from which a plant can learn its own figures.

The set of monitored parameters and their values differ for three reasons. The first is the type of receiver. The municipal network carries the effluent to treatment works with a biological stage, so it can take part of the load itself. A water body receives the discharge directly, and the requirements there are usually stricter. The second is what the plant makes: an electroplating shop will have metals on the list, a dairy will have fats and organics, and the two lists barely overlap. The third is the state of the particular network and how loaded it is: two neighbouring plants can be given different conditions simply because they discharge into different collectors.

The same document sets out the monitoring frequency, the sampling point (usually the control manhole at the plant outlet) and the sampling procedure. Samples are taken either by the receiver's own service or by an accredited laboratory; the plant's own monitoring is covered separately. An exceedance usually means not an immediate ban but a multiplier on the discharge charge — though with systematic exceedances the receiver is entitled to refuse the effluent.

Hence the practical conclusion: looking for "generally accepted" limit concentrations is pointless. The first action is to request your own discharge conditions from the receiver and obtain the list of parameters the site will be checked against.

What is measured

Suspended solids. Everything in the water that is not dissolved and is retained by the filter during analysis. The sources are raw material washing, sand and soil from the yards, mill scale, sludge, trimmings and product residues. For the receiver this is a direct mechanical load: solids settle in the collector, narrow its cross-section, wear out pumps and increase the volume of sludge at the treatment works.

BOD and COD. Both parameters describe organic pollution, but they measure different things. COD is the amount of oxygen needed to oxidise organics chemically, including the fraction bacteria do not break down. BOD is the amount of oxygen bacteria will consume over a standard period, that is, only the biodegradable fraction. The ratio between them matters more than the numbers themselves: if COD is noticeably higher than BOD, the effluent contains a lot of persistent organics, and biological treatment will not take it — a physico-chemical stage is needed. The sources of organics are sugars, proteins, fats, alcohols, and also dyes and solvents.

Fats and petroleum products. Fats come from food production, kitchens, dairy and meat processing. Petroleum products come from washing bays, car services, machine shops and stormwater from open yards. In the network both congeal on the walls, build up blockages and put pumping stations out of service; in a water body the film interferes with gas exchange.

pH. The acidity or alkalinity of the effluent. It rises and falls after CIP cleaning with alkalis and acids, pickling, electroplating and regeneration of ion exchange filters. Effluent that has moved in either direction away from neutral corrodes the concrete and metal of the collector and suppresses the biology at the treatment works.

Ammonium nitrogen and phosphates. Nutrients. Nitrogen comes with protein raw material and the products of its breakdown; phosphates come with phosphate-based detergents and with fertilisers. In a water body they act as fertiliser: the water blooms, oxygen is consumed, fish leave. At the treatment works they require a separate stage, so receivers watch them closely.

Temperature. Hot effluent comes from boiler houses, pasteurisation, container sterilisation and hot washing. Heat degrades seals and accelerates corrosion of the network, upsets the regime of biological treatment and changes conditions in the water body.

Surfactants — synthetic surface-active agents, the basis of detergents. The sources are laundries, washing of containers and equipment, CIP. They foam in channels and pumping stations, and at the treatment works they stop particles from sticking together and settling, reducing the performance of settling tanks.

Heavy metals — copper, zinc, nickel, chromium, lead, and also iron. They come from electroplating, pickling, metalworking, and from mine and quarry water. Biology does not break them down: metals pass straight through the treatment works and settle in the sludge, after which it can neither be composted nor hauled away as ordinary waste. That is why the requirements on them are as a rule the strictest on the list.

Parameter, source, treatment stage

How the parameters are brought down

Local treatment plants are assembled in the same order every time, and skipping stages does not work.

Equalisation. Effluent is uneven: a shift, a wash at the end of the day, a slug discharge from CIP. Every subsequent stage is set up for the average composition and lets a peak slug pass through. An equalisation tank with mixing evens out both flow and concentration, smoothing pH and temperature along the way. This is the cheapest stage and at the same time the one where cutting costs most often wrecks the line.

Mechanical treatment. Screen, sieve, grit trap. They take out coarse matter and sand, protecting the pumps and the equipment downstream.

Chemical treatment. The coagulant neutralises the charge of fine particles and they stick together; the flocculant gathers them into large flocs that can then be separated. The reagents are fed by a dosing station, and the dose is selected by jar testing on the actual effluent rather than from a table — every production has its own composition.

DAF flotation. Air is dissolved in water under pressure; when the pressure is released it comes out as a cloud of fine bubbles, they attach to particles and lift them to the surface, where a scraper skims the layer off. Flotation takes what settles poorly: fats, petroleum products, emulsions, fibre, light suspended solids, and with them part of the BOD and COD. The HLDAF flotation range in the catalogue covers from 4–5 to 95–100 m³/h.

Settling. A lamella clarifier works through a pack of inclined plates: solids settle on them and slide down into the sludge hopper. It takes out heavy solids and the flocs left after chemical precipitation of metals, it has no moving parts inside, and it takes up less space than a horizontal settling tank. The HLLC range runs from 3 to 120 m³/h.

Sludge dewatering. Both the flotation unit and the clarifier produce watery sludge that still has to be hauled away. The HLDS screw dewaterer accepts dilute sludge straight from the clarifier, without a separate sludge thickener; a chamber filter press works in cycles and gives a cake of lower moisture content. The stage seems optional right up to the first invoice for hauling liquid sludge.

Assembled together, these stages make up a line of industrial wastewater treatment — the composition is selected according to the list of parameters that need to be brought down.

What next

The order of actions before choosing equipment:

• ✓request the discharge conditions from the receiver and obtain the list of monitored parameters with the values for your own site;
• ✓have your own effluent analysed at an accredited laboratory against that list;
• ✓take the sample at the hour of peak load and separately after washing, not at a quiet moment of the shift;
• ✓measure the actual flow and its unevenness — daily volume, hourly maximum, volume of slug discharges;
• ✓record the effluent temperature at the hottest point of the cycle.

With these data, sizing becomes arithmetic: the flow sets the model size, the list of parameters sets the set of stages. Without them any scheme remains guesswork, and rebuilding a finished line costs more than the analysis and measurements at the start.