Different treatment processes will produce different operating effects, and the operating costs will also be different. The traditional activated sludge process and its deformation process are mostly used in large sewage treatment plants, while the oxidation ditch process is mostly used in small sewage treatment plants. The capital construction cost of the oxidation ditch process is lower than that of the traditional activated sludge process, but its operating cost is higher than the latter. The smaller the sewage treatment plant is, the more obvious the advantage of using oxidation ditch process is.

When the source water quality is controlled and the water is fed continuously with high load, the process will be complicated. when the content of industrial wastewater is high, the treatment will be more difficult and the treatment cost will increase.

The sewage treatment plant is a large consumer of energy consumption. through the description and analysis of its various costs, this paper illustrates the importance of saving energy and reducing consumption to the sewage treatment plant, and gives detailed cost control measures.

Cost composition of sewage treatment plant

1.1 Direct costs

(1) Energy costs: including electricity and water charges, of which electricity is the main cost, accounting for about 4050% of the total cost.

(2) material cost: including flocculant fee, laboratory fee, low-value consumable goods, etc.

(3) Direct labor and welfare expenses: wages and welfare expenses of all production personnel.

1.2 Manufacturing expenses

Including maintenance fee, raw material fee, spare parts fee and so on.

1.3 period expenses

Various expenses incurred by the management for the organization and management of production, including administration, departmental management expenses, financial expenses, equipment depreciation and other indirect expenses.

1.4 Analysis of cost composition of sewage treatment

The main results are as follows: (1) the power consumption and medicament consumption in the production cost are the expenses that must be incurred in the production process, and they account for a large proportion, which must be strictly controlled in the production process in order to reduce the cost effectively.

(2) the salary and welfare of personnel in production cost is the driving force to improve the enthusiasm of employees, enhance the vitality and cohesion of enterprises, and further do a good job in saving energy and reducing consumption, and it is also a symbol of the level of enterprise and social development. Under the condition of normal production operation, the higher the proportion, the better the development of the enterprise.

(3) on the basis of the sustainable development of the enterprise, the manufacturing cost should reduce the investment of maintenance, overhaul and fixed assets as much as possible so as to maximize the profit of the enterprise.

(4) the management fee is a part of the operating cost of the enterprise, which should be controlled to reduce unnecessary expenditure and reduce cost as much as possible.

II. Cost calculation of sewage treatment plant

2.1 electricity cost

The most energy-consuming equipment of water treatment enterprises includes various types of water pumps, oxygen supply equipment such as blowers and sludge thickening and dewatering equipment. To calculate the unit consumption of sewage treatment, on the one hand, it can be analyzed according to the number of equipment and power, on the other hand, combined with the statistical analysis of the unit consumption of sewage treatment in previous years, and with reference to the relevant local standards, to formulate a reasonable unit consumption of sewage treatment suitable for the enterprise.

For example, in the assessment standard for the operation and management of a sewage treatment plant in Jiangsu Province, there are clear assessment requirements for the unit consumption of sewage treatment. The assessment of the unit consumption of first-class A standard sewage treatment is divided into four levels: ≤ 0.34 degrees / cubic meter, 0.34 degrees / cubic meter-0.37 degrees / cubic meter, 0.37 degrees / cubic meter-0.42degrees / cubic meter and ≥ 0.42degrees / cubic meter. The first-level B standard is assessed according to 70% of the first-level A standard unit consumption, and those lower than the first-level B standard are assessed according to 50% of the first-level A standard unit consumption.

Therefore, the normal power consumption index of secondary sewage treatment enterprises is within the range of 0.17 kilowatt-hours and 0.42 kilowatt-hours per cubic sewage, according to the current electricity price of 0.852 yuan / kilowatt-hours (taking Nanjing as an example), then the power consumption cost can be calculated as 0.145-0.358 yuan per cubic sewage.

2.2 Pharmaceutical cost

Most of the agents in sewage treatment plants are polyacrylamide used in sludge dewatering machine or ferric trichloride and calcium oxide used in sludge plate-frame filter press for dewatering. According to statistics, the sludge output of sewage treatment enterprises produces 1 ton ~ 1.2 tons of dry sludge for every 10000 tons of sewage. The pharmaceutical costs of different sludge treatment equipment are as follows:

(1) the agent used in the sludge dehydrator is cationic polyacrylamide, which contains 0.6% polyacrylamide per ton of dry sludge, and the price is 25000 to 35000 yuan per ton. calculated at 30000 yuan per ton, the cost of flocculant is 0.018 to 0.0216 yuan per cubic sewage.

(2) the dosage of ferric trichloride used by the plate and frame filter press is 10% of the absolute dry mud, and the unit price is calculated according to the market price of 750 yuan / ton (ferric trichloride content is 40%), then the cost of ferric trichloride is 0.0075-0.009 yuan per cubic sewage. The dosage of calcium oxide used in the plate and frame filter press is 30% of the absolute dry mud, and the unit price of calcium oxide is 450 yuan / ton (purity 70 80%, granular specification 90 100 mesh). The addition cost of calcium oxide is 0.0135 to 0.0162 yuan per cubic sewage. Therefore, the cost of using medicament for sludge dewatering of plate and frame filter press is 0.021 to 0.0252 yuan / cubic sewage.

2.3 overhaul cost of facilities and equipment

It mainly refers to the special fund for the major repair of fixed assets such as facilities and equipment of sewage treatment plants. According to the construction, the investment per ton of water under construction of the sewage treatment plant is generally 1500-2000 yuan, calculated according to the conventional fixed asset formation rate of 85%, combined with the actual overhaul fee in the past and referring to the relevant data, the annual overhaul cost is extracted according to 1.7%-2.0% of the fixed assets, then the fixed asset repair cost of the sewage treatment enterprise is 0.0594-0.0931 yuan per cubic meter of sewage.

2.4 maintenance cost of facilities and equipment

It is mainly used for spare parts of fixed assets, low value and easy consumption, and regular maintenance and repair expenses of fixed assets, including maintenance of automatic control system, strong inspection of measuring instruments, preventive test of electrical equipment and some materials used in daily maintenance. According to the built, the investment per ton of water under construction is generally 1500-2000 yuan, calculated according to the conventional fixed asset formation rate of 85%, combined with the actual overhaul fee in the past and referring to the relevant data, if the annual maintenance cost of facilities and equipment is extracted according to 0.5%-1.0% of the fixed assets, the maintenance cost of sewage treatment enterprises is 0.0174-0.0466 yuan per cubic meter of sewage.

2.5 Disinfection cost

It refers to the disinfection of treated effluent from sewage treatment plants. At present, there are a variety of disinfection methods, such as ultraviolet lamp tube disinfection, single potassium persulfate disinfection powder disinfection, and so on. Taking potassium persulfate disinfectant as an example, according to the use method of mono potassium persulfate disinfectant, 0.5g disinfection powder per ton of sewage needs to be added to meet the process requirements, and the price of disinfection powder per ton is about 115000 yuan. The cost of disinfection is 0.046 to 0.0575 yuan per cubic meter of sewage.

2.6 sludge disposal cost

At present, there are many methods of sludge disposal in the country, such as landfill, composting, drying, incineration, making building materials and so on. According to the relevant data, the cost of sludge disposal is about 300-1000 yuan / ton. according to the sludge output of sewage treatment enterprises, 1 ton ~ 1.2t dry sludge is produced for every 10000 tons of sewage, and the cost of sludge disposal is 0.030.12yuan per cubic meter of sewage.

2.7 staff wage and welfare costs

According to the examination standard of operation and management of urban sewage treatment plants in Jiangsu Province, the labor staffing index of urban sewage treatment enterprises with secondary treatment is generally 2.5 million 30 people / (10,000 m3 / d). The larger the scale, the smaller the index. The smaller the scale, the larger the index. The staff salary is based on the average wage of the local staff and workers. If the average wage level of the staff and workers announced by the Nanjing Bureau of Statistics in 2011 is 54713 yuan / year / person, the staff wage cost can be calculated to be 0.037-0.449 yuan per cubic meter of sewage.

2.8 Enterprise management cost

The office and service expenses incurred by a sewage treatment enterprise for organizing and managing the enterprise. Calculated according to 10% of the staff wage cost, the management fee cost can be calculated to be 0.0037-0.0449 yuan per cubic meter of sewage.

2.9 other costs

Including sewage treatment enterprises for flushing all kinds of structures, pool surface, sludge treatment equipment water consumption, sewage, sludge test fees, greening fees, etc., the cost is about 0.010 to 0.015 yuan per cubic meter of sewage.

According to the above costs, it can be calculated that the sewage treatment cost of secondary sewage treatment enterprises basically changes in the range of 0.3665 to 1.2093 yuan per cubic meter of sewage, which is mainly due to the different sewage treatment scale, treatment process and disinfection methods of sewage treatment enterprises.

The above sewage treatment costs are calculated according to a certain electricity unit price, pharmaceutical unit price, disinfectant unit price and personnel wages, and these unit prices constantly change with the price level, so it is necessary for the sewage treatment plant to establish a corresponding adjustment mechanism. according to the author's work, it is considered that the above-mentioned costs can be divided into variable costs and constant costs, and the corresponding proportion can be calculated and adjusted when the above unit prices change in a certain range. For example, the price adjustment formula Pn=P0*K (K is the price adjustment coefficient is established.)

Khama1 * (b2/b0) + a2 * (c2/c0) + a3 (d2/d0) +. + an

Among them, A1, a2 and A3 are the proportion of variable cost, an is the ratio of immutable cost, b2, c2 and D2 are the changed prices of the relevant unit price, and b0, c0 and d0 are the original calculated prices of the relevant unit price. According to the above price adjustment formula, the sewage treatment cost can be adjusted according to the corresponding unit price change in a certain period.

III. Cost control of sewage treatment plants

Cost control is a complicated work, involving every worker in the unit. The key is to establish a concept of frugality, followed by a strict system and effective measures. Sewage treatment is a public welfare cause, and it does not produce any economic benefits (unless there are reclaimed water reuse and sludge utilization projects, but most plants do not). It is purely social and environmental benefits and high cost. therefore, cost control is the key to maintain the healthy operation and rapid development of the sewage treatment plant. In the daily operation and management, the author thinks that the following points should be done well.

3.1 Power consumption control

At present, the power consumption of the sewage treatment plant has reached more than 7 billion Kw.h/a, and shows an increasing trend, so it is the top priority to control the power consumption, and the submersible pump and blower are the key control points.

(1) A 100000-ton sewage treatment plant uses more than 10 submersible pumps, so it is of great significance to save energy and reduce consumption by controlling the pumps. The submersible pump should be opened with soft start, which can effectively reduce the power consumed by the excessive current during start-up. In addition, the submersible pump should be opened at a high liquid level, which can reduce the lift of the submersible pump and save energy.

(2) use variable frequency adjustable blower. It can be said that the consumption of the blower accounts for about 40% and 62% of the total power consumption, so it is very important to save energy on blast aeration. The influent quality of the sewage treatment plant is changing all the time, and more dissolved oxygen is needed when the water quality is poor, but not too high when the water quality is good. If the dissolved oxygen value remains unchanged (that is, the opening amount of the blower is unchanged), energy is wasted, and too much dissolved oxygen for a long time will even cause activated sludge to disintegrate and affect the effluent effect. The sewage treatment plant using frequency conversion regulation can automatically adjust the opening degree of the fan according to the setting value of the aeration tank on-line dissolved oxygen meter, so as to avoid unnecessary waste. In addition, the sewage treatment plant with staged influent can also stop aeration or weak aeration during the process of stopping the influent, because the stopping of aeration for 12 hours will not have a great impact on microorganisms.

(3) the sewage treatment plant with sectional electricity price can use water intake in different periods (such as valley, peak, peak, flat period) to avoid the peak period of electricity consumption, which can effectively reduce the electricity charge.

(4) continuous operation of some equipment, such as coarse grille, fine grille, sand suction machine and primary settling tank scraper, is of little significance to production, but it can shorten the service life of the equipment and increase power consumption and failure rate, so this kind of equipment can adopt intermittent operation mode. For example, the thick and thin grille can use the liquid level difference before and after the grid to control the start and stop; the mud scraper can be opened according to the initial settling sludge concentration, such as opening the valve when the precipitated sludge concentration reaches 5%; other equipment can be opened regularly, and so on.

3.2 Pesticide control: reduce the consumption of medicament as much as possible

The dewatering performance of the sludge produced by the sewage treatment plant, especially the activated sludge, is generally poor, and it is usually necessary to pretreat the sludge (that is, quenching and tempering) to improve its dewatering performance.

Polyacrylamide (commonly known as PAM), a commonly used organic polymer flocculant for quenching and tempering, reduces the dosage as much as possible on the premise of meeting the requirement of moisture content of dehydrated mud cake. The dosage is related to the properties of sludge, environmental factors and the type of dewatering equipment. In the actual operation, it is necessary to synthesize the above factors to find the best dosage that not only meets the requirements but also reduces the cost of adding drugs.

Generally speaking, in order to keep the sludge fresh, the fresher the sludge is, the better the treatment effect is and the less the drug consumption is; to control the source water quality, when the water is fed continuously with high load, the drug consumption will increase, and when the industrial wastewater content is high, not only the drug quantity will increase, but also the difficulty of treatment will increase; to control the sludge concentration, when the sludge concentration is 1.9-2.5%, it is the best, too low and too high is not conducive to treatment. To control the ratio of excess sludge to primary settling sludge in mixed sludge, the lower the ratio, the better, but in practice, it is difficult to do so, because there is often more excess sludge. In practical work, operators should constantly explore the law, accumulate experience, avoid unfavorable factors and reduce drug consumption.

3.3 Control maintenance costs

After a certain number of years of operation of the sewage treatment plant, the equipment maintenance appears to be particularly frequent, especially the imported equipment, whose expensive maintenance cost plays an important role in increasing the operating cost. In order to reduce the maintenance cost, we should start from the following aspects:

(1) strengthen the inspection of the equipment to achieve early fault detection and early maintenance, so as to avoid greater losses.

(2) do well the daily maintenance and maintenance of the equipment, reduce the failure rate of the equipment as far as possible, and prolong the service life of the equipment.

(3) to control the cost of overhaul and renovation. Comprehensively evaluate the feasibility of overhaul and renovation costs, the integrity of technical content, the reliability of maintenance quality and the comparability of economic value, determine the construction plan, and implement standards and quality control and strict acceptance and cost approval to reduce costs.

(4) the maintenance work order transmission system shall be implemented in maintenance. After the failure of the equipment, the personnel on duty should fill in the detailed maintenance work order, determine the maintenance items, budget with the relevant departments, determine the implementation plan, supervise the maintenance process, and sign the maintenance results. it can not only improve the maintenance efficiency but also reduce the maintenance cost.

(5) maintenance statistical analysis. This paper makes a comprehensive analysis of the maintenance plan and implementation, makes a comprehensive comparison between the maintenance effect and cost, and determines the matters needing attention in the future maintenance work, so as to reduce the maintenance cost.

3.4 reduce material cost

The purchasing department can make on-the-spot investigation to the suppliers or ask them to quote the supply price, and after comparing the price, the purchasing department can determine a number of suppliers that meet the procurement conditions. In this way, the supplier has a stable sales volume, while the enterprise can get a lower purchase price; the enterprise has a stable supply channel and supply, which is helpful to shorten the procurement cycle and save the procurement workload, reduce inventory and reduce the occupation of funds. Reduce the cost of capital occupation.

3.5 Control of labor costs

In order to give full play to the due reward and restraint role of the wage system, enterprises should formulate a more scientific salary and bonus system, pay more attention to the contribution to the enterprise in the distribution of wages and bonuses on the principle of giving priority to efficiency and giving consideration to fairness, establish and improve a scientific work performance evaluation system, match the income of employees with the contributions of enterprises, and fully mobilize the enthusiasm of workers. The establishment of a scientific evaluation system can also enable employees to have a more objective understanding of their own abilities, help to have a stable state of mind, put more energy into work, improve work efficiency, and effectively save labor costs.

3.6 Control, management and other expenses

Although the cost accounts for a small proportion of the cost, the effective saving can exert an imperceptible influence on reducing the cost and improve the effectiveness of cost control.

(source: water treatment tribe)