Engineering design of reverse osmosis used in coal chemical industry reuse water treatment

The main water source of the reuse water treatment unit in a coal chemical plant is the effluent from the sewage treatment unit, the sewage from the circulating water field and the sewage from the desalted water station, the combined treatment process of homogenization tank, high efficiency clarifier, multi-media filter, ultrafiltration and reverse osmosis is adopted, and the effluent is used as make-up water for circulating water field.

The reuse water treatment unit has been built and operated for nearly a year. The operation results show that the unit operates stably and the effluent quality meets the quality standard of reclaimed water used as make-up water for circulating cooling system in code for design of wastewater recycling engineering (gb50335-2002).

The make-up water volume of circulating water field in a coal chemical plant accounts for about 70% of the total fresh water consumption, if fresh water is used to make up water, it will cause a great waste of water resources. 
Based on this, a coal chemical plant in China adopted a combined process of homogenization tank, high efficiency clarifier, multi-media filter, ultrafiltration and reverse osmosis to treat and reuse the wastewater.
We have designed and produced wedge wire screen, rotary drum screen and sieve bend screen for this filtration system.

The combination process of effective pretreatment (wedge wire screen type filter) + ultrafiltration (UF) + reverse osmosis membrane (RO) can not only remove pollutants efficiently and stably, but also avoid the pollution blocking of reverse osmosis membrane and ensure that the effluent meets the quality standard of reuse water. It has been applied in the engineering design of reuse water treatment.

• Design scale and inlet and outlet water quality
  
  The influent water sources of the water treatment unit of coal chemical plant are mainly divided into the following categories:
  1. The water quality of the effluent from the sewage treatment plant meets the first level discharge standard of integrated wastewater discharge standard (GB 8978-1996).
  2. Waste water from circulating water plant.
  3.Wastewater from desalted water station.
  The design scale of the reuse water treatment device is 160 m3/h, and the design effluent is required to comply with the water quality standard of the reclaimed water used as the make-up water of the circulating cooling system in the code for design of wastewater reuse engineering (GB 50335 – 2002).
  The water source of the reuse water treatment device is relatively complex, and the pollution of organic pollutants, microorganisms, biological slime, colloids, heavy metals, hardness and other substances is more complex and serious than that of the conventional pure water, which requires higher pretreatment effect before ultrafiltration.
  According to the above water quality characteristics, the combined process of homogenization tank + high efficiency clarifier + multi-media filter + ultrafiltration and reverse osmosis is designed.

• Process flow and main design parameters(main design structure and equipment parameters) Wastewater homogenization tank:
  The discharge of circulating water field and desalted water station in the influent water source are intermittent, so homogenization treatment should be strengthened. 
  The design size of the homogenization tank is 37.9m × 7.6m × 3.5m.
  The design size of the homogenization tank is 37.9m × 7.6m × 3.5m. The homogenization tank is equipped with a wastewater lifting pump, one for use and one for standby, with a flow of 160 m3 / h.
  Two submersible mixers with a power of 3 kW are provided.

  High efficiency clarifier
  High efficiency clarifier is a steel structure wastewater treatment equipment which integrates coagulation / reaction / sedimentation.
  The single treatment capacity is 160 m3 / h, the designed effluent SS is ≤ 20 mg / L, and the design size is d 6000 mm × 7000 mm.
  In front of the high-efficiency clarifier, a lime milk dosing device is equipped, with two concentrated slurry pumps, one for use and one for standby, and a single dosing amount of 0-3 T / h; a flocculant dosing device, two mechanical diaphragm pumps, one for use and one for standby, and a single flow rate of 0-120 L / h.
  Coagulant dosing device, 2 mechanical diaphragm pumps, 1 for use and 1 for standby, with single flow of 0-44 L / h.

  Multi media filter
  The multi-media filter is used to remove the suspended solids and colloids in the water, and the filter materials are anthracite and quartz sand.
  There are two medium filters, each of which is designed to have a capacity of 80 m3 / h and a filtration speed of 10 m / h, and air-water backwashing is adopted.
  The design dimension of multi-media filter is d 3200 mm × 4 200 mm.
  We provide customized production services (Johnson screen filter) for prefilters of multi-media filters.

  Ultrafiltration system
  In front of the ultrafiltration system, a disc filter is equipped with a treatment capacity of 160 m3 / h, designed effluent turbidity ≤ 3 NTU, filtering accuracy of 50 μ m, filter type: Israel arkal 3df-8. The designed recovery rate of ultrafiltration system is 90%. The designed water quality is SDI ≤ 3, turbidity ≤ 0.5 NTU.
  The net water yield of the unit is 144 m3 / h, adopting sfp2860 dowtm UF membrane module (external pressure hollow fiber membrane) produced by Dow company, with a single membrane area of 51 m2, a total of 60 pieces.
  The ultrafiltration system and the reverse osmosis system are equipped with a set of chemical cleaning device, including a set of cleaning security filter, with a capacity of 150m3 / (H · set), and filter element filtration accuracy of 5 μ M.
  Chemical enhanced backwashing system includes sodium hypochlorite dosing device, two electromagnetic driven diaphragm pumps, one for use and one for standby, with a single flow of 0-1600 L / h; acid dosing device, two electromagnetic driven diaphragm pumps, one for use and one for standby, with a single flow of 0-550 L / h; alkali dosing device, two electromagnetic driven diaphragm pumps, one for use and one for standby, with a single flow of 0-240 L / h.

  Counter-infiltration system
  A safety filter is set in front of the reverse osmosis membrane. The filter precision of the safety filter is 5 μ M. the shell is made of corrosion-resistant 316 stainless steel. The filter rod is made of polypropylene spray melting. The hole shape is cone-shaped, and the operating flow rate is ≤ 10 m3 / (M2 · H).
  The high-pressure reverse osmosis pump is a horizontal centrifugal high-pressure pump. The material of the overflow part is stainless steel, q = 150 m3 / h, P = 1.4 MPa.
  The designed water yield of RO unit is 108 m3 / h, and the recovery rate is 75%. The designed desalting rate in 1 year is ≥ 98%, and the designed desalting rate in 3 years is ≥ 97%.
  The reverse osmosis system is equipped with reductant dosing device, two electromagnetic driven diaphragm pumps, one for use and one for standby, with a single flow of 0-15.1 L / h; scale inhibitor dosing device, two electromagnetic driven diaphragm pumps, one for use and one for standby, with a single flow of 0-7.6 L / h.

• SummaryCombined with the complex water quality requirements of the water inlet and the water quality requirements of the reuse water users, the combined process of homogenization tank + high efficiency clarifier + multi-media filter + ultrafiltration and reverse osmosis is designed for the advanced treatment and reuse of the wastewater in the coal chemical plant.The operation results show that the operation effect is good, the desalting rate and recovery rate can reach the design standard, and the economic benefit is obvious, which proves that the selected process is feasible and economic.