How to solve the problem of excessively high main indicators in the sewage treatment system

Due to the complex and diverse composition of organic matter in wastewater, it is difficult to individually determine the quantitative values of each component. In practice, composite indicators are commonly used to indirectly represent the amount of organic matter in water. There are two categories of composite indicators that represent the organic matter content in water: those expressed in terms of oxygen demand equivalent to the organic matter in water, such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total oxygen demand (TOD); and those expressed in terms of carbon, such as total organic carbon (TOC).

For a given wastewater, these indicators generally have different values, arranged in the following order from highest to lowest: TOD > CODCr > BOD5 > TOC.

一、Excessive Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) refers to the amount of dissolved oxygen consumed during the biochemical oxidation process of organic matter by aerobic microorganisms under 20°C and aerobic conditions. It represents the oxygen required for stabilizing biodegradable organic matter in water and is measured in milligrams per liter (mg/L). BOD includes not only the oxygen consumed by aerobic microorganisms for growth, reproduction, or respiration but also the oxygen consumed by reducing inorganic substances such as sulfides and ferrous ions, although the latter usually accounts for a small proportion. Therefore, a higher BOD value indicates a higher organic matter content in the water.

To reduce the organic matter content and improve BOD levels in effluent, processes such as coagulation, flocculation, sedimentation, sand filtration, or activated carbon filtration can be employed. These processes promote the proper balance of “food” and organic matter by enhancing the rate of biological oxidation through appropriate aeration methods, where air is introduced into the effluent to increase the level of settleable solids, which can then be removed from the effluent through filtration or clarification.

二、Excessive Total Suspended and Dissolved Solids

The methods for reducing Total Suspended Solids (TSS) depend on the TSS and Total Dissolved Solids (TDS) levels in the wastewater and the required discharge standards. Common methods to reduce TSS include coagulation, flocculation, sedimentation, and sand or carbon filtration.

Reducing TDS is a more complex process. If the pollutants are metal-based, such as calcium, magnesium, or iron, simple chemical additives can be added during the clarification process to reduce these pollutants. If the ions are highly soluble, such as sodium or chloride, desalination or evaporation processes may be required.

三、Excessive Nitrates and Phosphates

When a significant amount of nitrates and phosphates are not removed from wastewater and are discharged into the local environment, they contribute to increased BOD, leading to extensive weed growth, algae and plankton blooms, deoxygenation of water bodies, harm to organisms, and potential oxygen depletion. These nutrients can enter wastewater streams through various means, including human and food waste, detergents, and pesticides.

To address excessive nitrates and phosphates in wastewater treatment systems, the following methods may be useful:

Nitrate removal: Nitrate can be removed through various methods, including ion exchange, reverse osmosis, or conventional biological treatment and denitrification.

Phosphate removal: Effective methods for removing phosphates from wastewater streams often involve coagulation/chemical precipitation, depending on the type of phosphate present.

四、Presence of Oil and Grease in Wastewater

Oil and grease are hydrophobic substances that tend to repel water and adhere to surfaces without water. The presence of a significant amount of oil and grease in wastewater can lead to clogged drains and pipes, damage to human health, and harm to aquatic organisms. Oil and grease are often introduced into wastewater as byproducts of food production, and their discharge is strictly regulated, prohibiting their release with sewage.

Wastewater containing oil and grease requires treatment using Dissolved Air Flotation (DAF), where air is dissolved under pressure into the water to remove oil. As the bubbles rise to the surface, they attach to the oil and grease, allowing them to be skimmed off from the top. Another method for successful oil and grease removal includes certain types of filters, such as ultrafiltration or activated carbon.