What are industrial discharges in water and its treatment

The discharges to water can be defined as intentionally or accidentally introduced into the water bodies pollutants. These discharges directly or indirectly alter and damage the quality of the water and, therefore, of the ecosystem, posing a threat to the communities of living beings that inhabit it.

There are three main types of spills :

• Agricultural or livestock.
• Urban.
• Industrial

But it is on the latter that we are going to focus our attention. The industrial effluents are those from industry and are the most dangerous and polluting.

The Ministry for the Ecological Transition of Spain has established a general classification of discharges that, therefore, can also be applied to industrial discharges. According to this, discharges can be classified as direct and indirect.

• Direct discharges: are those that are dumped directly into continental waters (river courses, transitional waters, coasts, lakes and lagoons, groundwater, etc.) or any other element belonging to the Public Hydraulic Domain (surface waters, river beds and lake beds and reservoirs. Private waters and marine waters are not included). Also included are those discharges that are injected directly into the ground and therefore pollute groundwater .
• Indirect discharges: are those that are made in continental waters or in other elements of the Public Hydraulic Domain through ditches, channels or systems for collecting waste or rainwater (storm tanks). In the case of groundwater, these are discharges that seep through the ground and the subsoil.

The law also contemplates some special cases, for example, discharges to irrigation canals are considered direct, since they are discharged into the water that belongs to the Public Hydraulic Domain. The discharges to the boulevards can be considered direct because they are discharged to their waters or to the riverbed when it is dry and indirect to the groundwater since they can infiltrate the soil. Finally, indirect discharges are considered to be made into surface waters that may affect the quality of the receiving waters.

Once we know what industrial discharges are and what their classification is, we are going to talk about the types of industrial discharges that exist. There is a great typology of industries and depending on the type these can generate more or less polluting waste than others. Next, we are going to explain the kind of waste that some of the main industries can generate.

• Construction: waste is rich in suspended solids, metals and can cause the pH of receiving waters to change.
• Mining: also produces suspended solids, heavy metals and can alter the pH. In addition, it generates organic matter and cyanides.
• Textile and leather: waste from these industries can contain metals such as chromium, tannins, surfactants, sulfides, colorants and dyes, fats, solvents, acids (acetic, formic, etc.) and suspended solids. Within these solids are synthetic textile fibers that are considered microplastics.
• Automotive: it is one of the most polluting industries. Produces oils, lubricants, paints, metals, shavings, fuels, and wastewater.
• Naval: mainly petroleum, chemical products, solvents and pigments or colorants.
• Steel: heavy metals, acids and bases, oils, shavings and solids.
• Inorganic chemistry: especially chemical substances such as halogenates (fluorides), residues with mercury (heavy metal), phosphorus, manganese, molybdenum, lead, silver, selenium, zinc and also other compounds such as cyanides, ammonia, nitrogenous compounds, acids and bases.
• Fertilizers: especially nutrients in the form of nitrates and phosphates
• Pulp and paper: suspended solids, bleaches (chlorine) and bases and other substances that can affect the amount of dissolved oxygen in receiving waters.
• Pesticides: they produce organic pollutants such as organohalogens or organophosphates, carcinogenic compounds, biocides, etc.
• Chemical fibers: oils, organic compounds and also substances that affect the amount of dissolved oxygen in the receiving waters.
• Paints, varnishes and dyes: metallic compounds such as some with zinc, chromium, selenium, molybdenum, titanium, tin, barium or cobalt among others.

Given that industrial discharges can be of very different types and of diverse composition, it is difficult to establish a general treatment that eliminates all waste. It is very important when defining the treatments to know the specific composition of each discharge, define the discharge points and establish the appropriate physical, chemical or biological purification methods.

Next, we are going to talk in a general way about the treatment of industrial discharges in water and what they consist of.

Pretreatment

Although sometimes it is not present, the pretreatment consists of eliminating a certain residual load to the industrial waters before receiving the treatments. It comprises at least one of the following phases:

• Homogenization: since the flow of wastewater can vary throughout the days, the resulting concentration of pollutants will not be the same in the wastewater at all times. The homogenization phase consists of the same, in homogenizing or equalizing the concentration of pollutants in the waters of the industry in question. For this, the wastewater is stored in agitation tanks for days until the flow rates equalize.
• Grinding: necessary to protect the facilities from the entry of large objects that can obstruct and hinder the treatments. Implemented in industries such as agri-food, textile or paper.
• Degreasing: consists of cleaning oils and hydrocarbons. This process is especially important in those industries dedicated to the manufacture of these compounds or those that have lubrication circuits or through which oily substances circulate.
• Sandblasting: this pretreatment is applied in industries or companies such as sandboxes, foundries or concrete mixers.

Primary treatment

It is mainly based on the application of physical-chemical treatments to industrial wastewater. It may appear as the main treatment, an intermediate stage, or as the final stage. Among the main treatments are:

• The precipitation of metals and toxic salts.
• The elimination of oils and suspended materials.
• Clarification, which consists of the reduction of organic matter.
• Other more generic treatments such as sedimentation, coagulation-flocculation, flotation and neutralization.

Secondary treatment

It is based on the use of biological methods to purify wastewater. In order to apply this treatment, the effluents or discharges must be biodegradable and their characteristics must be well known to avoid damage to biological reactors. Biological treatment is characterized by the following elements:

• Activated sludge.
• Bacterial beds.
• Aerated or mixed lagoons.

Tertiary treatment

When discharges contain non-biodegradable organic compounds or more complex substances such as solvents, aromatic hydrocarbons (benzene) or nitrogenous and phosphorous compounds, more specialized techniques are needed.

• Activated carbon adsorption of pollutants: by surface attraction the particles are adsorbed or retained on the carbon.
• Membrane separation: some molecules can be separated based on their size, shape, or molecular structure. Reverse osmosis and microfiltration and ultrainfiltration stand out.
• Ion exchange: consists of the exchange of pollutants for other ions such as Na ⁺ , H ⁺ or OH ^– that are present in a membrane. When the discharge passes through the membrane, the ions present in it are replaced by the pollutants in the effluent.
• Chemical oxidation: it is achieved with the use of oxygen, ozone, chlorine … and consists of the elimination of both biodegradable and non-biodegradable organic compounds.

Learn more about this topic in this other AgroCorrn post about the different types of wastewater treatment .

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