Sludge Treatment and Disposal Technology (3)

11. How Is the Amount of Sludge Estimated?

There are generally two methods for estimating the amount of sludge:

(1) Estimation based on sewage treatment volume and solid content. For example, if the average sewage solid content of a city is 0.02%, the daily treatment volume is 600,000 tons, and the dewatered sludge solid content is 20%, then the annual wet mud cake production is:

600,000×0.02%×360/20%=216,000 tons/year

(2) Estimation based on population. For example, if a city has a population of 2,400,000, the daily per capita sludge (dry) production is 50 grams, and the dewatered sludge solid content is 20%, then the annual wet mud cake production is:

2,400,000×50/1,000,000×360/20%=216,000 tons/year

12. What Is the Reason Why the Sludge Problem Has Begun to Be Taken Seriously?

The reason why the sludge problem has become increasingly prominent is that after a long period of exploration and testing, a batch of sewage treatment plants built in the early stage still have not found a good disposal solution, and the resources for stacking, abandonment and landfill are getting less and less. The supervision of environmental protection departments in various places has been strengthened, and the country's sewage treatment is developing and expanding at an unprecedented speed. The disposal of sludge has become a thorny issue.

According to the urban population base, even if only 100 million people's sewage is treated, 25,000 tons of sludge cakes with a solid content of 20% will be produced every day. If this part of the sludge cake is stacked at a maximum of 2 meters, 600 national standard football fields will be required every year. For cities, the surrounding land resources are already difficult to meet the needs. Therefore, the reasonable disposal of sludge must be carried out as soon as possible.

13. Is Sludge a Dangerous Solid Waste?

Sludge is different from other solid wastes in that it has the following main characteristics:

(1) High moisture content. When the moisture content is more than 70%, it is difficult to incinerate, the transportation cost is high, the stacking area is large, and direct landfill will cause the landfill site to be scrapped prematurely.

(2) High content of microorganisms and pathogens. If it is not treated, directly applied or discarded, it may pollute the food chain.

(3) Foul odor pollutes the environment. At the same time, it emits greenhouse gases (20 times that of carbon dioxide) into the atmosphere.

(4) Ultrafine powder. There is a great danger during thermal drying and processing.

(5) Contains heavy metals. If it is not used under control, it may pollute the land and cause irreversible degradation of arable land.

At present, the hazards of sludge are little known. It is often illegally used, resulting in excessive accumulation of heavy metals in the land, soil compaction, and inadvertent pollution and destruction of human living environment and food.

14 Is Sludge Useful?

The main components of sludge are organic matter and minerals, so it can be used to improve soil.

The organic matter in sludge has a certain calorific value (2200-3000 kcal/kg·dry sludge), so after treatment, it can be used as a low calorific value fuel.

Since it contains a large amount of inorganic matter, it can also be used as a raw material for building materials after treatment.

There is no doubt that the above three existing utilization methods all require that these utilizations are economically feasible and hygienically safe.

15. What Is the Difference Between Sludge Treatment and Disposal?

Sludge disposal refers to giving sludge a final destination: either it is applied to farmland, greening and other soils as fertilizer and becomes part of the soil; or it is used as a resource to form useful materials, such as paving debris, cement, bricks, etc.; or it is landfilled and wastes land resources.

Any process that cannot achieve final placement can be considered as treatment. For example, sludge composting can only produce safe fertilizer effects after killing bacteria and maturation; incineration will eventually produce ash, which accounts for more than 40% of the original dry matter mass, so landfill or utilization must also be considered; drying is to remove most of the water in the mud cake, save transportation costs, reduce land occupation, lower landfill fees, and provide reduction, sanitation and economic conditions for other final disposal solutions.