Wastewater analysis and wastewater monitoring

Wastewater is generated through use, as rainwater from paved surfaces, and through foreign water, which enters the sewer system due to structural damage. In Germany, this wastewater is almost entirely treated in sewage treatment plants before being discharged into receiving water bodies. Contaminants can be present in both dissolved and undissolved forms, as well as in organic compounds.

Inflow to sewage treatment plants: Monitoring and analysis

The analysis of the wastewater entering sewage treatment plants is necessary to determine baseline values for nutrient content, organic load, and salt loads. Different measurement techniques from various manufacturers are used for this purpose. To obtain reliable measurement results, the quality of the water sample (sample filtration) is crucial. Depending on the contamination, the fineness of the filtration (membrane separation limit) can have a significant impact on the analysis results.

Wastewater components: A categorization

• Depleting substances: These include biologically degradable substances like uric acid and glucose. They can cause odors and reduce oxygen levels in water bodies.
• Nutrients: Particularly nitrogen and phosphorus compounds, which contribute to the eutrophication of standing water bodies and promote algae growth.
• Pollutants: These include toxins, heavy metals, synthetic organic substances, as well as bacteria, fungi, and viruses, all of which pose health risks.
• Disruptive substances: These include salts, fats, oils, as well as clays and sand.

Key parameters of wastewater analysis:

Chemical and Biological Oxygen Demand (COD, BOD)
The Biological Oxygen Demand (BOD) quantifies the amount of oxygen required to biologically degrade organic substances in the water. This measurement is taken under defined conditions over a specified time period. BOD serves as an indicator of wastewater pollution.

Adsorbable Organic Halogens (AOX)
The AOX indicator is a collective parameter in chemical analysis used to assess water quality and sewage sludge. It measures the total amount of organic halogen compounds that can be adsorbed onto activated carbon. This includes compounds of chlorine, bromine, and iodine.

Ammonium and Ammonium Nitrogen
Ammonium nitrogen refers to nitrogen present in the form of ammonium ions. This value is particularly relevant in wastewater analysis and treatment.

In agriculture, various nitrogen-containing fertilizers are used, including ammonium compounds and nitrates. Additionally, organic nitrogen forms are also present in these fertilizers. In the soil, ammonium is converted to nitrite and then to nitrate through the process of nitrification, involving bacteria. These nitrate compounds can be absorbed by plants. Organic nitrogen in the soil can also be mineralized and eventually transition into nitrate in soil humus. These compounds can also enter sewage treatment plants via rainwater or wastewater.

Nitrate and Nitrate Nitrogen 
Nitrates are salts and esters of nitric acid, and nitrate nitrogen is one of the inorganic forms of nitrogen found in wastewater. It is produced as the end product of the biological degradation process of nitrification, where ammonia is oxidized by bacteria. During this process, organic nitrogen sources such as urea and proteins are broken down into nitrate nitrogen.

The primary sources of nitrate nitrogen in wastewater are urea from feces and proteins. Microorganisms break down these substances through a multi-step process known as nitrification, converting them into nitrate nitrogen. This degradation process passes through the intermediate stages of ammonium nitrogen and nitrite nitrogen. Particularly protein-rich wastewater, such as from large kitchens, restaurants, and the food industry, contributes to nitrate pollution in municipal wastewater systems.

In the final biological treatment stages of sewage treatment plants, nitrate nitrogen is converted into harmless, elemental nitrogen, which then escapes into the atmosphere. This process, known as denitrification, is carried out by specific bacteria under low-oxygen conditions. It is important to note that if this treatment stage fails, significant amounts of nitrates can enter the environment.

Phosphate
Phosphates are salts and esters of orthophosphoric acid, and in a broader sense, the condensates of this acid and its esters are also considered phosphates. They are predominantly found in fertilizers and in detergents and cleaning agents. Through agricultural use and household wastewater, especially from phosphate-containing detergents, they enter the soil and sometimes the groundwater. Additionally, orthophosphates are used in industrial process water (pipes) to prevent corrosion.

In sewage treatment plants, measuring phosphate levels plays a key role in effective wastewater treatment. Phosphates can be removed from wastewater either through chemical precipitation or biological processes. In chemical precipitation, for example, dissolved iron salts are added to precipitate a large portion of the phosphate from the wastewater. This then settles along with other contaminants at the bottom of the primary sedimentation tank.

Given the limited phosphorus resources, recovering phosphates from wastewater and sewage sludge in treatment plants is gaining importance. This recovery helps promote the sustainable management of this vital resource.

Chlorides
Chlorides are chemical compounds in which chlorine is bonded to other elements, excluding oxygen and fluorine. Chlorine does not occur naturally in its elemental form but is always bound in compounds, often as metal chlorides like sodium chloride or cobalt(II) chloride, which are considered salts of hydrochloric acid (hydrogen chloride).

In the context of water and wastewater, chlorides are an essential inorganic anion. A noticeable concentration of chloride, often resulting from dissolved sodium chloride, can impart a distinct salty taste to the water. The chloride content in wastewater is typically higher than in drinking water.

Monitoring chloride levels is a crucial step in sewage treatment plants, particularly in coastal areas where seawater may come into contact with wastewater. In such cases, the chloride levels in wastewater can be especially high, making regular monitoring essential for efficient wastewater treatment in these regions.

Heavy Metals
Heavy metals such as nickel, copper, chromium, lead, zinc, and mercury are found in various wastewater streams. They primarily originate from industrial processes like electroplating and printed circuit board manufacturing, where they enter wastewater through washing and rinsing processes. However, heavy metals can also enter wastewater during metalworking and cleaning processes that use detergent-based complexing agents.

The standard methods for removing heavy metals from wastewater are based on converting the metals into a poorly soluble form that can be separated as a solid. The traditional method is precipitation using alkali or lime. Here, the pH level of the water is adjusted to a point where the heavy metals precipitate as poorly soluble hydroxides.

The efficiency of this process depends greatly on the pH level, which must be optimized for each metal. When multiple metals with different optimal precipitation pH levels are present, a compromise must be made, which can result in higher residual concentrations of metals in the wastewater.

To effectively remove heavy metals, especially when complexing agents are present, a two-stage chemical-physical treatment process is necessary. This process includes decomplexation and precipitation. After the addition of a decomplexing agent, a precipitant is added, which ultimately forms a poorly soluble metal sulfide. This settles at the bottom of the tank and can be removed along with other contaminants.