**8. Sewage pollution intensity**

The intensity of sewage pollution is called the power of sewage pollution or its concentration, and the more waste materials in the sewage, the stronger it is. Usually, the strength and weakness of wastewater in terms of organic matter in it are measured according to the following indicators:

### A. BOD5 (Biological Oxygen Demand)

BOD5 is the amount of oxygen needed by bacteria to decompose organic matter in wastewater. This standard is the most important tool for measuring biochemically degradable organic substances, which is commonly used in wastewater. The higher the oxygen required, the higher the concentration of organic substances in wastewater that can be oxidized by bacteria. It has been proved by experience that the BOD of a sample is different in hours and even in the early days, and today, at the global level, the value of this index within 5 days (BOD5) has been chosen as the standard. BOD5 relates the reduction of oxygen in the sample (which is consumed by aerobic bacteria in 5 days to decompose organic matter) with the concentration of organic matter that can be decomposed by bacteria, and its unit is mg/liter.

B. COD (Chemical Oxygen Demand)

Chemically required oxygen.

In this method, the amount of oxygen proportional to the chemical decomposition and stabilization of organic materials is called chemically required oxygen. This index is in milligrams per liter.

C. TSS (Total Suspended Solids)

Total suspended solids include materials such as organic particles such as plant roots, soil mineral particles, and plastic particles. Suspended solids are another


#### **Table 8.**

*Range of BOD, COD, and TSS changes in some countries (mg/liter).*

sign of wastewater quality. This index is expressed in milligrams per liter. The range of changes of the mentioned three indicators is different depending on the type of wastewater and its characteristics. **Table 8** shows the range of changes in BOD, COD, and TSS (mg/liter) in some countries.

In addition to the above guidelines, different steps have been taken in various countries in order to ensure public health and protect the environment, in the planning of wastewater application in agriculture, an example of which is the development of microbiological standards and guidelines. In terms of developing and applying such standards and guidelines, countries can be divided into several groups: (A) In industrialized and advanced countries such as America and France, standards and guidelines have been developed with a conservative point of view and based on advanced technology and high cost, as well as with low risk tolerance. (B) In some other countries, WHO guidelines, which are based on low technology and low cost, are accepted and form the basis of control [31, 32] and (C) In contrast to the above countries, the third group of countries, which mostly include developing countries, have accepted very strict standards without study and planning. Although these standards are accepted by legal authorities and are very good for having an international image, they are practically unacceptable and unenforceable due to economic and technical reasons. For such countries, perhaps the best solution is the step-by-step development of standards, which, if implemented, can effectively prevent health risks. In relation to the country of Iran, the environmental standards for the use of wastewater are very advanced and include a total of more than 50 physical, chemical, and microbiological parameters, which is ideal if implemented. But according to the existing economic and technical conditions, there are major problems as follows that do not make the implementation of the standards practical:


and total fecal forms, and other important parameters in agriculture are not measured. Therefore, such statistics cannot be a suitable basis for evaluating the quality of wastewater for use in agriculture. It should be remembered that strict standards and advanced technology do not necessarily mean reducing environmental risks and risks associated with wastewater. Because in many cases, the lack of proper operation of sewage treatment plants, the lack of sufficient funding for the management of treatment plants and the application of appropriate monitoring systems, or the lack of enforcement of laws, cause advanced technology and developed standards to lose their practical meaning. And as a result, environmental risks increase [31, 32].
