The sewage from the municipal sewage treatment plant refers to the solid, semi-solid and liquid wastes produced after sewage treatment. It contains a large amount of organic matter, heavy metals, pathogenic bacteria and pathogenic bacteria, etc., without any treatment, it will cause serious damage to the environment. Pollution. The overall requirements for sludge treatment are stabilization, decontamination and reduction. There are many methods for sludge treatment and disposal at home and abroad, generally using concentration, digestion, dehydration, drying, effective use (mostly for agricultural use), landfill and incineration, or a combination of several methods. It should be said that the treatment and disposal of sludge from sewage plants is quite different from that of advanced countries.
The stabilization and dewatering of sewage sludge (generally dewatering to a moisture content of 70% to 80%) is usually referred to as sludge treatment; composting, landfilling, drying, heat treatment and final utilization of sludge are called Disposal of sludge. If the toxic and harmful substances in the dewatered sludge exceed the agricultural standards, sanitary landfill and sludge drying and incineration technologies should be considered. From the development of foreign sludge treatment, the control of sludge for farmland is becoming more and more strict in Europe, Japan or the United States, and the proportion of sludge drying and heat treatment is increasing year by year.
1. Treatment of sludge
The sludge stabilization treatment has aerobic stability and anaerobic stability. Aerobic stabilization has many advantages, but the energy consumption is very high, and it is only used when the amount of sludge is small. The anaerobic stabilization of sludge is usually carried out by a medium temperature (35 ° C) anaerobic digestion method. At present, more than a dozen large-scale sewage treatment plants have adopted this method. After the sludge is digested, the organic matter content is reduced, the performance is stable, the total volume is reduced, and a large amount of biogas is generated during the sludge digestion process (digestion degradation of 1 kg COD can produce 350 L of biogas) recycle and re-use.
However, due to the complicated process of the digesting device, the one-time investment is large and the operation is difficult. The cost of sludge anaerobic digestion and biogas utilization equipment accounts for about 30% of the investment and operation costs of sewage treatment plants, and most of them require imported technology and equipment. From the actual operation of the surveyed digester, only a few achieved the expected results. There are management and design issues, as well as the economic and safety issues of biogas utilization.
In addition to the anaerobic digestion of sludge, the sludge stabilization problem should be combined with the scheme of reducing sludge generation and stabilizing mud in the sewage treatment process. For example, in the design of sewage treatment process, the wastewater aeration time is extended to reduce the sludge production; the sludge mud age (such as the mud age of more than 20 days) is increased in the design parameters, and the sludge tends to be stable to the sewage treatment process. For small and medium-sized sewage treatment plants, it is advisable to use a treatment process with a delayed aeration function (such as a treatment process such as oxidation ditch). Some sewage treatment processes have low investment (such as section A of the AB method), and the amount of sludge is large, which increases the treatment cost of sludge.
The stability of the sludge does not mean that the sludge is harmless. The farmland also needs to meet the national standard for the control of pollutants in the agricultural use. Among them, the requirements for cadmium, mercury, arsenic, benzopyrene and polychlorinated biphenyl are relatively high, and the nature of sludge should be controlled by strictly controlling the discharge of industrial wastewater sources.
In terms of sludge stabilization, in addition to biological methods (including medium temperature digestion, high temperature digestion and utilization of microorganisms and certain additives), chemical methods are also used, and some dehydrated sludge is added with hydrochloric acid to adjust the pH to 2~ 3, the reaction for 60 minutes followed by the addition of sodium nitrate; some added lime to the dewatered sludge. The latter is used more in Europe.
2. Disposal of sludge
(1) Compound fertilizer
For most sewage plants (especially a large number of small sewage plants), sludge is a feasible and realistic solution for farmland. The nitrogen, phosphorus, potassium and trace elements in the sludge increase the yield of crops; the organic matter and humus in the sludge are good soil amendments. The sludge is properly concentrated, dehydrated and transported to the suburbs or neighboring provinces as agricultural fertilizers, which is the method adopted by many sewage plants. However, fertilization in farmland is seasonal. When mud is not needed, the sewage plant will be muddy and affect normal operation. Therefore, some sewage plants pay the fees, so that the farmers can pull the sludge away without asking where it is going, which will cause secondary pollution.
(2) Sanitary landfill
This method covers a large area and has a large amount of operation. In the rainy season, the sludge is more difficult to compact, and the site needs to be selected after the use period. The use of sludge sanitary landfill for large-scale sewage plants is a last resort. The cost of sanitary landfills is not low. There are also biogas safety collection systems for sanitary landfills in foreign countries. There are special requirements for layered and covered soil, drainage and greening. In view of the rising land prices and the smell of landfills, in recent years, regardless of the EU countries or the United States, Japan, the proportion of sludge sanitary landfill is getting smaller and smaller, and the existing landfills in the United States will be gradually closed.
Some cities intend to ship sewage sludge to urban landfills for disposal. There are two practical problems: First, management system problems. The layout, design and investment of the waste transfer station and landfill are managed by the Environmental Sanitation Bureau, while the sludge of the sewage treatment plant is managed by the municipal system. The design of the landfill site and the distance of the site are not considered to accept the sludge from the sewage plant; The dewatered sludge has a high moisture content. The sludge transported to the landfill requires a moisture content of not more than 30%. At present, the dewatered sludge moisture content of the sewage plant is 70% to 80%. Such sludge is not easily crushed and landfilled unless the sludge is properly prepared. Dry or add lime and flocculant treatment. Regardless of the landfill, the sludge should be treated with a high dryness dewatering scheme.
(3) Drying and incineration
Since the combination of sludge drying and sludge incineration has less one-time investment than single sludge incineration and low treatment cost, sludge drying is often a pre-treatment of incineration. The sludge drying can control the moisture content of the sludge to 10% to 40%, reduce the volume and weight of the sludge, reduce transportation costs and landfill fees, and greatly reduce the odor of the sludge.
The drying device is divided into direct drying and indirect drying, and its energy consumption is related to sludge composition and moisture. Indirect drying (using biogas through a heat exchanger) is generally recommended with a vertical drying unit and a fluidized bed process. In the tandem process of drying and incineration, the degree of drying depends on the calorific value of the sludge and the heat energy of the recovery incinerator, so that the energy of the drying is balanced as much as possible without additional fuel. The design and drying of sludge in Shanghai Shidongkou has high calorific value and energy balance. Sludge fluidized bed incinerator, the temperature is above 800 °C, the sand is recycled in the furnace, and the external exhaust gas should be properly treated.
If dewatered sludge is incinerated together with garbage, the foreign experience is to add 15% to 20% of sludge with a water content of 30% per ton of waste. The drying and incineration of sludge is likely to be the development direction of large sewage treatment plants in some big cities. Of course, due to the strict requirements for dust removal from incinerators abroad, in addition to the use of electric dust removal, it is necessary to cool down and heat, add acid and alkali to achieve smokeless dust emissions.
(4) Comparison of landfill and incineration
Comparing the schemes of sludge sanitary landfill and incineration disposal, the main process flow is:
Original sludge→concentration→digestion→dehydration→sanitary landfill
Original sludge → concentration → (digestion) → dehydration → incineration → incineration ash landfill
For the incineration treatment process, in order to avoid the reduction of the calorific value of the sludge after digestion, it is also possible not to carry out sludge digestion and disposal. The economic comparison results of the above two processes, regardless of the use of domestic equipment or imported equipment, the disposal costs of the two are basically the same. The sludge is disposed according to domestic equipment, and the operating cost is converted into sludge dry solids. The total cost of treatment is about 800 yuan/t. The 10,000 m3/d wastewater treatment plant produces 2 tons of DS, and the treatment cost per ton of sludge is about 0.16 yuan, which is compared with the domestic sewage treatment plant sewage treatment cost (excluding depreciation and repayment interest) 0.3-0.45 yuan/m3. The cost needs to be increased by 35% to 50%, which is equivalent to the foreign example.
Since the engineering cost and operating cost of sludge sanitary landfill and sludge incineration are roughly equal, the incineration scheme has obvious advantages from the harmlessness and reduction of sludge. This is also the reason why sludge incineration has developed rapidly in foreign countries (especially in Western Europe and Japan). The sludge in the Netherlands is 100% disposed of by incineration. After incineration, a small amount of sludge can be used for concrete, brick products, aggregates for roadbed pavement and backfill for engineering construction.
3.Sludge treatment and disposal should pay attention to the problem
(1)When designing urban sewage treatment facilities, industrial pollution source control, sewage treatment process and sludge treatment and disposal should be considered as a system. Based on the analysis of the sludge composition and quantity of the sewage treatment plant in the city, it is correct to comprehensively plan the sludge disposal, coordinate the multi-party relationship, and implement the phased implementation plan. The sludge treatment of the sewage plant is not seriously disposed, and the sewage treatment is incomplete.
(2) For the final disposal of sludge, our lack of experience, such as the drying of sludge and the design of incinerators, is much more complicated than the sewage treatment process. We should seriously study, introduce, digest and absorb foreign technology and experience. It is necessary to formulate and improve regulations and standards related to sludge disposal. The sludge digestion and biogas utilization devices of some sewage treatment plants in China are basically imported technology and equipment from abroad.
(3) Use multiple ways to solve the sludge problem. The big factory is different from the small one, and the south and the north are different. In some places, sludge can be combined with garbage disposal, and some local sludge can be combined with coal-fired power plants. For the sludge of a small sewage plant with a large amount of land, it is necessary to summarize the experience of agricultural use. In the sludge drying and heating technology, various solutions such as solar energy, microwave and wet oxidation can also be considered. Some sludge can be treated together with sludge in industrial wastewater treatment. It is necessary to be good at summing up the experiences and lessons of various places and vigorously developing localized equipment. This is a sunrise industry. It is necessary to actively explore low-cost, harmless and multi-purpose sludge treatment and disposal methods suitable for national conditions.