COD removal is a constant challenge in wastewater treatment. There is a lot to consider around the discharge standards as well as the treatment processes. Learn more about chemical oxygen demand and its treatment options.
COD removal in wastewater
What is COD?
The abbreviation COD stands for “chemical oxygen demand”. It is a sum parameter for all substances dissolved in wastewater – both organic compounds and compounds that are difficult to degrade. Thus, in the field of wastewater treatment, it indicates the degree of pollution. The chemical oxygen demand is only a theoretical value. It indicates how much oxygen would be necessary to break down all pollutants.
The COD serves as the basis for determining the performance of a wastewater treatment plant. As a rule of thumb, the more pollutant load that enters the wastewater, the more organic compounds there are. Microorganisms are needed to break down these compounds. These respire some of the pollution, which allows them to grow and multiply. Oxygen is necessary for this process of nourishment. The more organic compounds there are in the wastewater, the more oxygen the microorganisms need to break them down.
The sewage treatment plant therefore has the task of saturating the wastewater with oxygen and supplying the microorganisms with it. Laboratory analyses determine how high the chemical oxygen demand is. The duration and intensity of aeration must then be adjusted. Only with a sufficient oxygen supply do the microorganisms can respire the dirt load. Otherwise, COD removal would only take place partially.
What are the effects of too high COD on wastewater?
In the environment, a complete reduction of COD is usually harmful, as numerous living organisms require a certain level of pollution as a basis for life. However, too high COD could lead to oxygen depletion, as the microorganisms extract oxygen from the water to respire the pollution. This can lead to the extinction of living organisms. Maintaining a balanced ratio is therefore crucial.
The challenge here, however, is that oxygen, which is a natural component of the air, is not sufficient to supply the wastewater with the amount of oxygen needed to bring about adequate wastewater treatment. An artificial supply of oxygen is therefore essential for COD removal.
Which processes are best suited for COD removal?
The chemical oxygen demand is not only defined by biodegradable substances – whether dissolved or undissolved – but also by substances that are difficult to degrade. These can be cadmium or copper, for example. Depending on the region-specific regulations and discharge plans for the wastewater, it must be treated differently. For example, indirect discharge into a municipal sewage treatment plant is always contrasted with direct discharge into a receiving water body.
Both types of discharge require a different COD removal. While a municipal wastewater treatment plant usually offers to treat COD that is difficult to break down, direct discharge requires strict compliance with regulations. The limits for metals can be very strict, which is why additional processes must be applied in addition to biological wastewater treatment.
One such process is dissolved air flotation – a chemical-physical process. Based on the total of all dissolved and non-dissolved substances, various flocculants in different concentrations are suitable for the treatment. The flocculants dissolve the dissolved substances out of the wastewater and bind them together to form flocs. The system supersaturates the wastewater with oxygen through pressure. When the plant releases the pressure, the excess oxygen dissolves and rises in ultra-fine bubbles at a speed of about four metres per hour. The bubbles lift the bound flocs and flush them out as sludge.
Another process is electrical oxidation. This uses diamond-coated electrodes that act as anode and cathode. The wastewater is stored in a tank in which the electrodes are immersed. By supplying electrical energy, oxidative and thus strongly disinfecting OH radicals are produced. These treat the substances that are difficult to degrade and prepare the wastewater for direct discharge.
Controlled COD removal requires a high level of experience in wastewater treatment. You can always rely on the quality of the advice offered.
