Two Ways For Sludge Process

We use tap water every day,after used, the sewage enters the sewage treatment plan. When the sewage becomes clear, the pollutants form sludge. How to deal with so much sludge?
For the time being, There are 2 ways for sludge treatment,including  incineration and fertilizer production.
In some cities where heavy industrial enterprises gather, they can only dry the sludge and make into fuel.
In the central area of the city, there is no heavy industry. All sewage sources are domestic sewage. The sludge produced is basically free of heavy metal elements and rich in organic substances such as nitrogen, phosphorus and potassium. The sludge of urban sewage treatment plants is strictly controlled and standardized. After high temperature aerobic biological fermentation, it has met the relevant standards.
The moisture content of sludge is about 80%, it is too high for the ecological fertilizer treatment plant. First, it is dehydrated to 55% of moisture content; then send the sludge rotary dryer for further drying, after sludge dryer, the water content is reduced to less than 40%.
 After the sludge fermentation treatment is converted into organic fertilizer, it is rich in humic acid, organic matter and nutrients, which can effectively improve soil fertility. Compared with the current situation that the soil organic matter content in China is declining, such improvement is more urgent.