Often the sewage plant sludge stabilization and dewatering (generally dewatered to a moisture content of 70% to 80%) called sludge treatment; sludge composting, landfill, drying and heating treatment and final use, known as sludge disposal. If the toxic and hazardous substances in the dehydrated sludge exceed the agricultural standard, we should consider sanitary landfill and sludge drying and incineration technology. From the point of view of the development of foreign sludge treatment, whether in Europe, Japan or the United States of America for agricultural sludge control is becoming increasingly strict, while the proportion of sludge drying and heating treatment is increasing year by year.
1. Sludge treatment
Sludge stabilization treatment has aerobic stabilization and anaerobic stabilization, aerobic stabilization has many advantages, but the energy consumption is very high, only when the amount of sludge is small to adopt. Sludge anaerobic stabilization treatment usually adopts medium temperature (35 ℃) anaerobic digestion method. Domestic has more than a dozen large-scale sewage treatment plant using this method, sludge digestion, organic content, stable performance, total volume reduction, sludge digestion process also produces a large amount of methane gas (digestion degradation of 1kgCOD can produce 350L methane gas) can be recycled.
But due to the complexity of the digestion device process, a large one-time investment, the operation is difficult. Sludge anaerobic digestion and biogas utilization device costs, accounting for about 30% of the investment and operating costs of sewage treatment plants, and most of the technology and equipment to be imported. From the investigation of the actual operation of the digester has been built, only a few to achieve the expected results. There are management, design problems, but also biogas utilization of economic and safety issues. Better, such as Tianjin Dongjiao Sewage Treatment Plant, the plant's design scale for the treatment of municipal sewage 400,000m3 / d, sludge production 2460m3 (96% moisture content), generating 13,300m3 of biogas for four 248kW generators to generate electricity, power generation can be 27,000 degrees per day, and grid-connected with the municipal power supply.
Sludge stabilization, in addition to anaerobic digestion of sludge, should be combined with sewage treatment process to consider less sludge and stabilize the mud quality of the program. For example, sewage treatment process design to extend the sewage aeration time, reduce the production of sludge; design parameters to increase the age of sludge (such as sludge age of 20 days or more), try to stabilize the sludge sewage treatment process. For small and medium-sized sewage treatment plants, the use of sewage treatment process with delayed aeration function (such as oxidation ditch and other treatment processes) is desirable. Some sewage treatment processes have low investment (such as the AB method of A section), while the amount of sludge is more, increasing the cost of sludge treatment. Therefore, sewage treatment and sludge treatment should be considered in a unified manner, together with the calculation of investment and operating costs.
Sludge stabilization is not equal to the sludge is harmless, used for agricultural land also need to comply with national standards on sludge agricultural use of pollutant control standards limits. See the table below. One of the cadmium, mercury, arsenic, benzo(a)pyrene, polychlorinated biphenyls (PCBs) requirements are relatively high, should be through strict control of industrial wastewater sources of discharge to control the nature of sludge.
Foreign countries in the sludge stabilization, in addition to biological methods (including medium-temperature digestion, high-temperature digestion and the use of micro-organisms and certain additives), but also used the chemical method, some will be dewatered sludge with hydrochloric acid to adjust the pH value to 2 ~ 3, the reaction for 60 minutes and then add sodium nitrate; some of the dewatered sludge to add lime. The latter is applied more in Europe.
2. Disposal of sludge
(1) Compound fertilizer
According to China's current economic conditions, most of the sewage plant (especially a large number of small sewage plants), sludge used for agricultural land is a more feasible and realistic program. Nitrogen, phosphorus, potassium and trace elements in the sludge, crop yield; sludge in the organic matter, humus is a good soil conditioner. Sludge transported to the outskirts of the city or neighboring provinces as agricultural fertilizer after proper concentration and dewatering is a method used by many sewage plants. However, agricultural fertilizers are seasonal, and when sludge fertilizers are not needed, sewage plants will be full of sludge, affecting normal operation. So some sewage plants pay farmers to haul away the sludge without asking where it goes, which can cause secondary pollution. You can also view more technical documents from China Sewage Engineering Network.
The Beijing Municipal Academy of Environmental Sciences and the Beijing Academy of Agricultural Sciences cooperated in a trial of sludge from a sewage treatment plant in Beijing's Miyun County, which was processed into a composite fertilizer through composting for use in farmland. The plant handles 15,000m3 of urban sewage per day, with a sludge production of 5-6t/d (80% water content), and the sludge is of good quality due to the acidification- aerobic sewage treatment process. A certain amount of N, P, K was added to make a compound fertilizer (the ratio of N, P, K was 1:0?9:0?4), and directly granulated as sludge granular fertilizer. Through the winter wheat field test in Panggezhuang, Daxing County, Beijing, as well as in the greenhouse in the rape and corn seedling potting fertilization test, have achieved promising results. As it is made into granular sludge fertilizer, it is easy to transport and store.
(2) sanitary landfill
Shanghai municipal sludge disposal of sewage plants put forward the "treatment of a little, landfill a little, the use of a little" principle, the Shanghai Municipal Water Affairs Bureau of the organization of sludge treatment, disposal and use of thematic studies, put forward the sludge used as farmland, sanitary landfill and sludge incineration point of the layout and specific Phased implementation program to prevent secondary pollution. This is undoubtedly the right move.
Shanghai Bailonggang large-scale sewage plant, according to the sanitary landfill requirements for the construction of sludge landfill, according to the nature of sludge, moisture content and mechanical properties and other factors to design. Landfill plant life of seven years, the bottom of the landfill with a blind tube will leachate back to the sewage plant treatment. This method occupies a large area, the operation of the workload, when the rainy season sludge is more difficult to compact, to the use of the term still need to choose another site. Sludge sanitary landfill for large sewage plants, is a last resort expedient. The cost of sanitary landfill is not low, foreign landfill sanitary also have methane gas safety collection system, the layering of soil and drainage, greening have special requirements. In view of rising land prices and landfills have odor, in recent years, whether the EU countries or the United States, Japan, the proportion of sludge sanitary landfill is getting smaller and smaller, the United States has a landfill will be gradually closed.
Some cities (such as Chengdu) proposed to transport sewage plant sludge to the city landfill and disposal, there are two practical problems: First, the management system on the problem. Garbage transfer station and landfill sites, design and investment, is the Bureau of Sanitation management, while sewage plant sludge is municipal system management, the design of the landfill life and distribution distance is not considered to accept the sewage plant sludge; Second, the dehydration of sludge with high water content. The sludge transported to the landfill is required to have a water content of not more than 30%, while the current water content of the dehydrated sludge from sewage plants is 70% to 80%, and this kind of sludge is not easy to be crushed and landfilled, unless the sludge is properly dried or treated with lime and flocculants. No matter what kind of landfill, the sludge should take high dryness dewatering program.
(3) drying, incineration
Domestic in recent years in some major cities have been built and are building a number of municipal waste incineration sites. But the sewage plant sludge for incineration disposal, only Shanghai Shidongkou sewage treatment plant (design scale of 400,000m3 / d) with sludge incinerator device, is scheduled to go into operation at the end of this year. Incinerator using foreign technology in the domestic manufacturing, sludge drying and incineration equipment for a total investment of RMB 80 million, the cost is not high.
Because sludge drying and sludge incineration combined with a single sludge incineration than a one-time investment is less, low treatment costs, so sludge drying is often incineration of pre-treatment. Beijing Qinghe sewage plant phase II project and Tianjin Xianyang Road sewage plant, the proposed first sludge drying device. Sludge drying can make the sludge moisture content control in 10% ~ 40%, reducing the volume and weight of sludge, reducing the transportation costs and landfill costs, and sludge odor is greatly reduced.
The drying device is divided into direct drying and indirect drying, and its energy consumption is related to sludge composition and moisture. Indirect drying (the use of biogas through the heat exchanger) is generally recommended vertical drying device, and the choice of fluidized bed process. Drying and incineration tandem process, the degree of drying depends on the calorific value of the sludge and the recovery of incinerator heat, so that the drying of the energy as much as possible balanced, without adding additional fuel. Shanghai Shidongkou design sludge drying and incineration, sludge with high calorific value, energy balance with surplus. Sludge fluidized bed incinerator, the temperature of 800 ℃ or more, the furnace has sand recycling, the external gas to be properly treated. Sludge incinerators are far simpler than waste incinerators, and sludge incineration does not produce dioxins. The picture below shows the sludge incinerator and the dust removal device for incineration ash at the Colombes sewage treatment plant next to the Seine River in Paris, France.
If the dewatered sludge and garbage and incineration, foreign experience is that each ton of garbage to add 15% to 20% moisture content of 30% of the sludge. Sludge drying and incineration, may be some large cities, large sewage treatment plant development direction. Of course, because of foreign incinerator dust emission has strict requirements, in addition to the use of electric dust removal, but also cooling and heating, acid and alkali, to achieve the emission of soot-free.
(4) landfill and incineration comparison
Shanghai and Zhejiang, some units have made a sludge sanitary landfill and incineration disposal program comparison. The main process is:
Raw sludge→concentration→digestion→dehydration→sanitary landfill
Raw sludge→concentration→(digestion)→dehydration→incineration→incineration ash landfill
For incineration process, in order to avoid the reduction of calorific value of digested sludge, it can also be disposed of without sludge digestion. The economic comparison results of the above two processes, regardless of the use of domestic equipment or imported equipment, the two disposal project costs are basically the same. According to the domestic equipment for sludge disposal, operating costs folded into the sludge dry solids, the total cost of treatment is about 800 yuan / t. 10,000m3 / d sewage plant produces 2 tons of DS, per ton of sludge treatment cost of about 0?16 yuan, with the domestic large-scale sewage treatment plant sewage treatment costs (not counting the depreciation and repayment of interest) 0?3 ~ 0?45 yuan / m3 compared to the need to increase the cost by 35% to 50%!
Since the cost of sewage treatment (including depreciation and interest on loan repayment) is 0.3 to 0.45 yuan/m3, it is necessary to increase the cost by 35% to 50%, which is comparable to foreign examples.
Since the sludge sanitary landfill and sludge incineration of its engineering and operating costs are roughly equivalent, then, from the point of view of the harmlessness of the sludge and the reduction of the incineration program has obvious advantages. This is also abroad (especially in Western Europe and Japan) sludge incineration development faster. The Netherlands sludge is 100% incineration disposal. After incineration, a small amount of sludge can be used for concrete, masonry products, road base pavement aggregate and engineering construction of backfill.