Sewage treatment process

1. Project Overview

In order to meet the environmental protection requirements of Shaanxi Forest Industry Workers' Hospital, a sewage treatment project is planned to be implemented. The MBR process is selected as the sewage treatment process, which integrates biodegradation, sedimentation, and filtration into one, reducing the number of treatment facilities, lowering operating costs, reducing failure points, and saving land area. Construct a sewage treatment plant with a processing capacity of 360m3/d.

2. Process flow

The hospital sewage enters the regulating pool to regulate the water quality and quantity. Install a grille in the regulating pool to intercept large floating debris in the sewage, effectively reducing the treatment load and providing assurance for the long-term normal operation of the system. The sewage is lifted to the MBR reactor by the regulating tank lifting pump. The MBR reactor is equipped with hollow fiber membrane modules, and the sewage is immersed in the membrane modules. Through the suction of the self-priming pump, the pollutants in the sewage are removed by the negative pressure inside the membrane fiber cavity. After ultraviolet disinfection, the effluent meets the standard for discharge.

3. Process Introduction

3.1 Grille

One equipment grille is used to intercept larger floating objects to ensure the normal operation of subsequent equipment. The slag in the grille needs to be cleaned regularly and can be disposed of with garbage. The grille material is stainless steel.

3.2 Design of regulating pool

One regulating pool, used for regulating water quality and quantity, fully underground steel-concrete structure. Two lifting pumps are installed in the regulating pool, one in use and one as backup, and the start and stop of the lifting pumps are controlled by a liquid level switch. Technical parameters of water pump: flow rate: 15m3/h, head: 22m, power: 2.2kW. Set up a liquid level switch in the pool.

3.3 MBR membrane bioreactor

The sewage is pumped into the MBR reactor by the regulating tank lift pump. Hollow fiber membrane modules are placed inside the MBR, and the sewage is immersed in the membrane modules. Through the suction of the self-priming pump, the negative pressure inside the membrane fibers is used to operate. The material of the membrane module is polyvinylidene fluoride (PVDF), with a nominal pore size of 0.2 μ m, which is an effective barrier for suspended solids, colloids, and other substances; Hollow fiber membrane fibers are relatively thin, have good flexibility, and can maintain a long service life. Even if there is a phenomenon of membrane fiber damage, due to the inner diameter of the membrane fiber being only 270 μ m, it can be quickly blocked by sludge and has no impact on the treated water quality.

Set up two cleaning tanks in the sewage treatment plant to regularly clean and maintain the membrane components. This project adopts blower aeration, which not only provides the necessary dissolved oxygen for microbial growth, but also cleans the surface of membrane fibers with rising bubbles and the turbulent water flow generated, preventing sludge aggregation and maintaining stable membrane flux. The designed air-water ratio is 25:1. The remaining sludge generated in MBR is minimal and is regularly discharged into the sludge tank. Three HC-801S fans, two in use and one backup, parameters: air volume Q=3.33m3/min, air pressure 0.4kgf/cm3, power P=5.5kW. MBR effluent is pumped by a self-priming pump to the UV sterilizer, and a glass rotor flowmeter is installed on the outlet pipe of the self-priming pump to facilitate observation of water volume and membrane component blockage. Two sets of float level gauges are installed inside the MBR, with high water level starting the pump and low water level stopping the pump. The MBR reactor has a processing capacity of 360m3/d, a box size of 5.0m × 4.0m × 4.0m, an effective water depth of 3.5m, and uses a stainless steel combination water tank.

3.4 UV sterilizer

Ultraviolet disinfection is the use of ultraviolet radiation with a wavelength range of 200nm to 275nm to irradiate various bacteria, viruses, parasites, algae, and other pathogens in water. After a certain dose of ultraviolet radiation, the DNA structure in their cell tissues is destroyed and becomes inactive, thereby killing bacteria, viruses, and other pathogens in water and achieving the purpose of disinfection, sterilization, and purification. Ultraviolet sterilization is fast, effective, and does not produce any secondary pollution. After multiple comparisons, this project adopts ultraviolet disinfection, using one ultraviolet disinfection device with an irradiation intensity of 25uw/cm2 to 30uw/cm2 and an irradiation time of 10 seconds.

4. Electrical control

The power line is introduced from the distribution room in the campus and supplied to the control cabinet in the sewage treatment plant control room through cables, with repeated grounding and grounding resistance meeting the standard requirements. The entire system is equipped with voltage loss protection to prevent the motor from starting automatically when suddenly powered on after a power outage. It also has an emergency stop switch in case of accidents and a fault alarm in case of abnormal situations. The power supply within the station adopts a radial approach to introduce various electrical equipment points to improve the reliability of power supply. At the same time, each equipment uses a single tube and single wire for power supply, and the signal line is completely independent of the power supply and distribution line, effectively avoiding interference between circuits and ensuring the normal, stable, and reliable operation of various sewage treatment equipment; At the same time, it provides convenience for the inspection and maintenance of future equipment circuits, and can be checked one by one according to the situation, without affecting the operation of other equipment in the sewage treatment plant, effectively ensuring the effectiveness of sewage treatment. All outdoor lines use famous brand cables from well-known domestic enterprises to ensure power supply safety and equipment operation requirements. An effective, stable, safe, and reliable control system plays a crucial role in the normal operation of the entire sewage treatment plant and directly affects the effectiveness of sewage treatment.

The main components used in the electrical control system of this sewage treatment plant are all imported equipment. A touch screen display with a fully Chinese menu is installed on the system control box, which can display the complete system process and equipment working status, realize monitoring, operation, control, and alarm of system equipment, and control the equipment at the same time as the buttons on the control box panel, forming a "double safety" for system operation to ensure its normal and reliable operation. According to the process requirements, the start and stop of the sewage lifting pump are fully automatic and equipped with automatic/manual control. When automatic control is selected, the entire processing process is controlled by the PLC based on the liquid level and time program. When manual control is selected, each device is manually controlled as needed.

(1) The operation of the entire processing system is mostly completed by PLC automatic control.

(2) The start and stop of the lift pump are controlled by the water level, with the pump starting at medium water level and stopping at low water level.

(3) The fan adopts automatic control and switches once a day.

(4) All types of electrical equipment are equipped with circuit short-circuit and overload protection devices to ensure the safe operation of electrical equipment.

(5) All power equipment is started directly.

(6) Imported products are selected for key components of self-control and electrical equipment.

5. Operation effect

The project has passed the acceptance of the environmental protection department.

6. Process characteristics and economic analysis

6.1 Process Characteristics

(1) The disinfection effect is good. Traditional treatment processes, due to the high concentration of suspended solids in the effluent, allow bacteria and viruses to adhere or wrap in suspended flocs and are not easily killed by disinfectants. In MBR processes, hollow fiber membranes are used as membrane components, which belong to the scope of microfiltration membranes. The membrane separation effect can effectively intercept bacteria in wastewater and reduce the amount of subsequent disinfectants; At the same time, due to the very low concentration of suspended solids in MBR effluent, bacteria and viruses lose their barrier and are more easily killed.

(2) The effluent quality is stable. Membrane bioreactor is a new sewage treatment process that combines membrane technology with traditional activated sludge biochemical treatment technology. It replaces the gravity settling process in traditional processes with membrane separation process, solving the problem of poor settling of solid particles in traditional processes, which leads to the loss of active microorganisms and ultimately results in system failure and unstable effluent quality. The sludge retention time and hydraulic retention time are completely separated. Membrane bioreactors have strong adaptability to changes in incoming water quality and operating conditions, and can ultimately complete the solid-liquid separation process well, ensuring stable effluent quality.

(3) High processing capability. The ultrafiltration membrane intercepts the vast majority of microorganisms, resulting in a rich variety and total amount of microorganisms in the reactor. Maintaining high MLSS and low F/M in the reactor can deeply oxidize organic matter and reduce the discharge of residual sludge. The reactor can simultaneously carry out nitrification and denitrification, successfully removing nitrogen. The sludge retention time is quite long, thus completely retaining the slowly growing nitrifying bacteria in the system.

(4) The process flow is simple. Traditional processes include physical and chemical, biological+physical and chemical, biological+biological+physical and chemical, and advanced treatment of activated carbon. Compared with physical and chemical methods, membrane bioreactors can save expensive drug and electricity costs; Compared with traditional biological treatment, the cumbersome units such as aeration tank, secondary sedimentation tank, and sludge concentration tank in the biological treatment process can be integrated into MBR tank, and then the membrane can be immersed in MBR tank for biochemical filtration and effluent. The process is simple and easy to manage.

(5) Save land. Due to the simple process flow, fewer processing units, and shorter hydraulic retention time compared to traditional activated sludge methods, the tank capacity is small and the structure is compact. Compared with the activated sludge method or contact oxidation method used in traditional water treatment, the land occupation is 1/4 to 1/2 of that of traditional water treatment.

(6) Easy to achieve fully automatic control. The whole machine can achieve automatic intermittent operation, and the system pump valve automatically starts and stops according to the program. It is equipped with control panels such as water level automatic control, membrane fouling control, and fault alarm, which are centralized and modular for easy observation.

(7) Low investment, low operation. Due to the small number of equipment and simple process, the one-time construction cost and long-term operating cost are lower than traditional sewage treatment facilities, and expensive drug costs such as flocculation are eliminated.

6.2 Economic Analysis

The total installed capacity of the MBR system is approximately 35.5 kW, with an operating power of 21.6 kW and an operating power consumption of 1.39 (kWh)/m3 [electricity cost is calculated at 0.85 yuan/(kWh)]. The operating cost is approximately 1.18 yuan/m3.