How to dispose of reverse osmosis concentrated water

The disposal of a mixture of reverse osmosis concentrated water and other water depends on the volume and quality of the concentrated water, the geographical environment of the disposal site, and the potential impact on water sources and soil. The current typical disposal methods are:

  • Surface water discharge (ocean, tide water);、
  • Deep well injection;
  • Sprinkler irrigation;
  • Wastewater treatment equipment;
  • Evaporation pond.

1 . Surface water discharge

When the concentrated water of the membrane system is discharged into the surface water body, the primary concern is the negative effect on the quality of the water body. The quality of the water discharged into the water body determines whether to treat the concentrated water of the system before discharge. Generally speaking, reverse osmosis concentrated water has low dissolved oxygen content, high hydrogen sulfide content and acidity. When the concentration of one or more common ions in the concentrated water is higher or lower than the receiving water, it is called “common ion toxicity”.

One way of indirect discharge into surface waters is to mix the concentrated water of the membrane system with treated sewage or rainwater and discharge it. If the concentrated seawater is mixed with 1000mg/l TDS of treated sewage in a ratio of 2:1, the TDS of the drainage will be reduced to be similar to the surrounding seawater. Concentrated water from brackish water systems mixed in similar proportions will also achieve a TDS similar to that of inland surface water.

If the water body at the outlet has a strong circulation capacity, the high TDS concentration of the concentrated water will be quickly dispersed, but if a large amount of concentrated water is discharged into lakes, lagoons, shellfish breeding seas or high-quality fish growing water bodies with less salinity , It will cause greater poisoning.

Surrounding conditions: As the receiving water bodies may include rivers, lakes, estuaries, canals, oceans and other water bodies, the surrounding conditions may vary greatly. The surrounding conditions include the bottom geometry of the receiving water body, the salinity, density and flow rate of the water. The salinity, flow and density of the receiving water body will vary with the depth of the water, the distance from the discharge point and the time.

Surrounding conditions: As the receiving water bodies may include rivers, lakes, estuaries, canals, oceans and other water bodies, the surrounding conditions may vary greatly. The surrounding conditions include the bottom geometry of the receiving water body, the salinity, density and flow rate of the water. The salinity, flow and density of the receiving water body will vary with the depth of the water, the distance from the discharge point and the time.

Discharge conditions: Discharge conditions include the geometry of the discharge facility and discharge flow conditions. The discharge facility can be a pipe or a longer multi-port diffuser. The discharge can be surface type or submerged type.

Drainage outlet structure: The structure of the drainage outlet should ensure that the mixing conditions are reached, and will not cause any damage to the receiving water body, such as aquatic life, wild animals and surrounding areas. Generally, a simple nozzle discharge is adopted, and the concentrated water is discharged into a large amount of receiving water under high turbulence to ensure dilution and mixing. However, in most cases, a carefully designed drain structure is used to improve mixing conditions. This design is called a diffuser, and some branch pipes are connected vertically on a straight pipe.

Characteristics and design variables of the diffuser:
The length of the diffuser tube;
The diameter of the diffuser tube;
The material of the tube;
Material of drain and valve;
The number of drains and valves;
The distance between the drain and the valve;
The angle between the drain and the main pipe.

2 . Deep well injection

Another method of concentrated water disposal is to inject the concentrated water into the ground through deep well injection. When conditions do not allow discharge to surface water, such as the acceptability of water bodies, the availability of cement, or management restrictions, it is a technically feasible method to discharge concentrated water into the ground through deep well injection. Deep well injection systems are generally used to treat concentrated water, sewage, and industrial and hazardous wastewater. Deep wells are classified according to the type of use and fluid injected.

Deep well injection is a simple and effective way to continuously dispose of a large amount of reverse osmosis concentrated water, and is not affected by weather conditions. But the process of implementing deep well injection is very complicated. The requirements for geographical conditions are quite special. The selected location must be separated from the aquifer suitable for drinking, so the injection position should be lower than all adjacent aquifers, and the permeability of the rock and soil should be relatively high to facilitate injection. Injection must not cause deterioration of water quality in the aquifer. During the construction of the injection well, the test workload is relatively large. It is necessary to ascertain the depth of the aquifer at the site, analyze the rock and soil structure, and conduct a pressure test. Reference to the geophysical records of the site will help to discover other factors that may affect the injection. Determining the injection well location requires the following tasks:

  • The research area must be seismologically stable. In the area affected by the earthquake and/or the downward slope of the injection well, the injected stratum structure is stable without faults.
  • In order to process a large amount of concentrated water under reasonable injection pressure, there must be a highly permeable and extensive injection area.
  • The injection area must contain saline with a TDS concentration of more than 10000mg/l.
  • The injection into the recipient water should have no economic impact, nor endanger mineral resources, nor affect the gas storage or fresh water storage system.
  • The closed injection bed must have sufficient thickness and be located in an impermeable gap.
  • The quality of the concentrated water should be compatible with the aquifer cement.

In order to determine the monitoring area, the enclosed bed, the amount of the injection formation, the nature of the aquifer, and the depth of the reinforcement borehole, it is necessary to collect the drilling and core of the test/intermediate test tunnel. After the test/intermediate test hole is completed, use logging equipment to perform geophysical logging records in the borehole. Geophysical logging records should at least include single-point electrical contact, gamma rays, temperature, flow meters, calipers, dual induction, borehole compensation sonic logging, etc.

The design of the injection system is based on the flow rate of the concentrated water. The flow rate can be adjusted by designing the retention system for the peak average flow rate. The size of the reinforced borehole is based on the daily average injection speed and pressure, the maximum working days and the peak hourly flow rate. Consider the injection pressure when designing the pump system. The pressure is related to the frictional resistance in the well and the viscosity of the injected fluid. In addition, in the feasibility decision, the density difference between the concentrated water and the injection layer liquid, and the pushing pressure (water delivery rate) of the bottom cave of the rock formation must be evaluated.

Compared with co-processing wastewater discharge liquid, injection wells have stricter design requirements for concentrated water treatment. This is due to the inconsistency of their industrial category. The basic difference in construction involves the well’s piping and gaskets.

The injection system should have the backwashing capability to remedy the failure. If the blockage is eliminated under pressure, backwashing can play a role in mechanical cleaning. At low flow rates, regular backwashing will increase the life of the well and reduce clogging.

Another consideration in the design of the injection well is the assessment of the quality of the concentrated water. This is to consider the compatibility of the reinforcement drilling type and the selected material. Concentrated water is moderately corrosive to steel, so it can cause crusts in well gaskets, drill reinforcement layers and rock formations. Concentrated water can migrate upwards, so the alarm system of the upper aquifer is protected by the network of monitoring wells, and the monitoring is generally located in the aquifer above the injection area to monitor the degradation of water quality. Usually, monitoring wells sample various parameters regularly to determine whether the injection wells are operating normally. It is very important for the deep well injection system to have backup disposal methods. The backup method is essentially a completely different disposal method, or another injection well, or some other form of permission for the disposal of concentrated water. Although deep well injection provides a viable alternative to ground wood discharge disposal, its cost is quite high. For concentrated water disposal, when there is no other acceptable method, the injection well method is used.

3. Wastewater treatment plant

Drain the RO concentrated water into the municipal sewage treatment system. Because too high TDS will adversely affect the growth of sludge, the TDS concentration of the concentrated water will be limited by the sewage treatment plant. Therefore, it is necessary to obtain permission from the municipal department and the sewage treatment plant before designing.

4. Sprinkler irrigation / land application

Concentrated water disposal can be completed by the application of land irrigation. Concentrated water is used for irrigation, because it temporarily supplements drinking water, groundwater or surface water used for irrigation, so it is also a method of water protection. Although this method of disposal may be beneficial, it is generally limited by the quality of the concentrated water. In most cases, concentrated water contains high concentrations of dissolved solids and chlorides, so it cannot be used for sprinkler irrigation unless blending or a large number of items are used for treatment.

Sprinkler irrigation is only used in a few applications. Such applications were initially used as intermediate research projects to assess possible environmental impacts, plant tolerance and feasibility of the overall plan.

Concentrated water can be pre-treated or blended with other water sources, such as treated discharged wastewater. Provide a usable irrigation water. The problems related to this disposal method mainly focus on the tolerance of flora, animals and/or crops, and the impact on groundwater and surface water systems. The restricted components in concentrated water are usually TDS, nitrides, radioactive elements and metals. Before implementing sprinkler irrigation schemes, the issues to be aware of are salinity, soil permeability, toxicity of individual ions, and the impact of component scaling.

It is necessary to evaluate the impact of sprinkler irrigation water on underground aquifers through infiltration and on surface water that may contain sprinkler irrigation water. Before the sprinkler irrigation research plan, the government management agency should participate in the establishment of the feasibility of the plan and the experimental draft.

An important consideration is that the irrigation water is required to be relatively close to the water source, and the implementation and storage of concentrated water spray on cloudy and rainy days must be considered. If storage in rainy days cannot be achieved, backup disposal methods must be used. With this method of disposal, there should be a monitoring well network system and regular testing of parameters to provide data on the basic water quality of groundwater and its impact on irrigation schemes.

5. Evaporation pond

Evaporation ponds are a relatively practical method of disposal in areas with high evaporation, low rainfall and enough cheap land. Evaporation ponds generally undergo leakage lining treatment and overflow treatment, so that they will not be discharged into groundwater and surface water bodies, so special approval is not required. However, it is necessary to detect and detect leakage and ensure that the overflow will not enter the groundwater. Evaporated residual solids should be cleaned regularly and sent to landfill for disposal.

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