Brine Treatment (ZLD)
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Brine can be identified as a liquid stream with increased salinity that is produced from many industrial processes and was considered for a long time only as a waste stream. The temperature of a brine stream can vary significantly depending on its source.
Sources of brine include:
- Mining Processes
In the desalination process we have a product and a by-product stream. The product, called permeate, is the pure water with some of the dissolved solids that pass through the process and together they compose the produced fresh water. Desalination brine is a by-product liquid stream with higher concentrations of most of the feed’s dissolved solids, some of the pretreatment additives (residual amounts of coagulants, flocculants, and antiscalants), organics, microbial contaminants and any particulates rejected by the RO membranes.
Supply demand during the last decade has risen for both for potable and industrial good quality water. With the decrease of freshwater sources, the increase of population and new advancements made in desalination technology, the water providers have turned to the treatment of brackish water (BW) and seawater (SW) to meet these demands
By 2007 the total produced water worldwide from desalination processes raised up to 47.6 Mm3/d and by 2015 to double with 97.5 Mm3/d and 45% of this production taking place in the Middle East. 70% of the desalination plants after 2000 were membrane processes, which brings up to date Reverse Osmosis (RO) to 63% of the total desalination operations. 23% are Multi Stage Flash (MSF), 8% Multi Effect Distillation (MED) and the rest are Electrodialysis (ED)/ Electrodialysis Reversal (EDR) and hybrids. Seawater RO (SWRO) can concentrate the salt concentration up to 2 times higher and MSF to 1.5 times. The brine produced by these processes, which is also called concentrate or reject, is mainly composed of a high concentration of Sodium Chloride (NaCl) and diverse other dissolved salts depending on feed water composition and process pre-treatment (Calcium, Magnesium, Bicarbonates, etc.).
Fig.1, Desalination industry by technology, users, and cost components (Costs assume a $0.05/kWh electricity cost and an oil price of $60/bbl), Desalination and sustainability - An appraisal and current perspective, Veera Gnaneswar Gude, Water Research 89 (2016)
Brine is a very loose term in the water industry. As per common RO membrane pressure resistance limitation, RO can only concentrate saline stream up to about 70,000 ppm (mg/L) Total Dissolved Solids (TDS) and can’t treat it further. In this text, we’re going to consider as brine all saline streams of 65,000-85,000 ppm that can’t be concentrated further with SWRO.
Brine disposal can prove to be quite problematic as,
- it increases the salinity of the receiving water bodies
- it impacts the local marine life
- it may contain pretreatment and membrane cleaning chemicals
- it may contain metals from the corrosion of the systems (Cu, Fe, Ni, Mo, Cr)
- it creates aesthetic issues (colorization)
- it impacts the nearby aquifers from leaks in the brine pipes
- it creates permanent damage due to the discharge infrastructure works.
Brine is either directly disposed of minimized before disposal but due to increasingly stricter government legislations, conventional brine management methods like surface/ deep water discharge, deep well injection or discharge to wastewater treatment plants may not be a feasible choice in a near future.
Brine quantity depends from the desalination plant’s production capacity and its recovery rate. The recovery is expressed as the percentage (%) of the freshwater produced flowrate to the total feed flowrate to the system. BWRO has recoveries from 50 to 90% and SWRO typically from 30 to 55%. Higher recovery results in smaller concentrate volume (higher salinity) and vice versa. The volume of brine produced by the desalination plant can be calculated as,
Vb = Vp x (1-R)/R (1)
- Vp = permeate volume
- Vb = brine volume
- R = system recovery rate (%)
Brine quality depends on:
- the feed composition and its salinity
- the desalination membranes’ salt rejection
- the total recovery
BWRO concentration factor is typically 4 to 10 while SWRO usually is 1.5 to 2.0 times. Brine TDS (TDSb) depends from the feed and permeate TDS concentrations (TDSf and TDSp) and the plant recovery (Y),
TDSb = TDSf x 1/(1-R) x (RxTDSp)/(100x(1-Y)) (2)
The concentration can be calculated as,
CF(%) = 1/(1-R) (3)
If the membrane salt passage (SP) is known, CF can be calculated as,
CF(%) = [1 – (R x SP)]/(1-R) (4)
SP (%) = 1 - % salt rejection = permeate TDS (TDSp)/feed TDS (TDSf) (5)
The salt CF is mainly limited by the brine’s increasing osmotic pressure (πbrine). For SWRO, this limit is ca. 65,000 to 85,000 mg/L. Optimum recovery for a single-pass SWRO system is 40 to 45% and the CF moves in a range of 1.5 to 1.8. For comparison BWRO plants typically have recoveries of 70 to 90% and concentration factors of 4 to 10.
Depending on the feed quality we can use the following rules to predict the brine quality
- brine pH is higher than the feed because it has higher alkalinity
- RO membranes reject heavy metals in a similar ratio as calcium and magnesium
- most organics are rejected in ≥ 95% (except for those with low molecular weight (MW))
- groundwater (GW) BWRO brine, may be anaerobic and may contain hydrogen sulphide (H2S)
If pretreatment is included in the desalination process, the RO feed water will have reduced levels of certain constituents such as dissolved metals, microorganisms, and particles but also slightly increased concentration of inorganic ions such as sulphate, chloride, and iron if coagulants are used. Brine may also contain residual organics from source water conditioning with polymers and antiscalants.
The generated brine has a low turbidity (usually < 2 NTU), low suspended solids (TSS) and biochemical oxygen demand (BOD) (typically < 5 mg/L), because most substances have to be removed in the pretreatment due to the sensitivity of the membrane process. But if the plant’s pretreatment side streams are mixed and discharged together with the brine, the mix may even have an increased turbidity, TSS and occasionally BOD.
Acids and scale inhibitors added to the feed water are rejected by the SWRO membrane and also affect the mineral content and quality of the brine. Scale inhibitor levels in the concentrate are usually < 20 mg/L.
Brine is also produced during many mining processes, such as oil and gas operations. In mining processes, large quantities of water are pumped in the ground in order to extract minerals. For example, in the oil mining operations, according to the American Petroleum Institute estimate, nine barrel of water are recovered for each barrel of oil during a typical extraction. Mining brine contains high salt content and dangerous chemicals, which could be very dangerous for marine life and disposing it is one of the biggest environmental challenges. Deep-well injection and surface storage in ponds are used to dispose of excess brine and are considered a major source of contamination of ground water. Mining brine that is discharged to the sea proved to be harmful to marine organisms.
Scaling species in RO plants are mainly calcium carbonate (CaCO3), calcium sulphate (CaSO4) and barium sulphate (BaSO4). Others such as calcium phosphate (Ca3(PO4)2), Silicate or metal scaling can also occur. In order to apply scale control, acid treatment and antiscalant dosage are used. In RO sulphuric acid was most commonly used but the use of antiscalants, such as polyphosphates, phosphonates or polycarbonic acids has become very common due to the negative effects of inorganic acid treatment
Table 1 ,Physical and chemical properties of brine from seawater desalination and the potential environmental/ecological impacts from its disposal.
|RO plants||MSF plants|| |
|Salinity and temperature||65-85 g/L at ambient SW ToC|| |
≈ 50,000 mg/L± 5-15 oC from SW ToC
↓ vitality & biodiversity at higher valuesharmless after good dilution
|Effluent density||> density of water||depends on the process||↓ biodiversity|
|Dissolved oxygen (DO)|| |
well intakes → typically < SW DOopen intakes → ≈ SW DO at ambient SW ToC
|may be < SW DO due to physical deaeration and use of oxygen scavengers||/|
Biofouling control additives and by-products
|Chlorine||neutralized before the membranes to prevent membrane oxidation||ca. 10-25% from feed dosage, if not neutralized|| |
↑↑ toxic for many organismsdegrades rapidly
|Halogenated organics||usually < harmful levels|| |
varying composition and concentrationstypically trihalomethanes
carcinogenic effectsdispersal with current and thorough evaporation
Scale control additives
|Antiscalants||< toxic levels||< toxic levels|| |
poor/ moderate degradability + high total loads→ accumulation, chronic effects, unknown side-effects
Foam control additives
|Antifoaming agents (e.g. polyglycol)||Not present (treatment not required)||< harmful levels|| |
Contaminants due to corrosion
|Heavy metals||↑concentrations of iron, chromium, nickel, molybdenum if ↓ quality stainless steel is used||↑copper and nickel concentrations if ↓ quality materials are used for the heat exchangers|| |
Copper- MSF (15-100 mg/L)- ↓ toxicity for most species; ↑ (bio)accumulation and long term effectstraces metals → no toxic or long term effects (except maybe for Ni in MSF)
The five conventional brine management options in the United States are used in the following capacities (Table 2 & Fig.3),
- surface water discharge (45%)
- sewer disposal (27%)
- deep-well injection (13%)
- land application (8%)
- evaporation ponds (4%)
Table 2, Most common brine disposal methods in the United States
Brine disposal method
Principle and description
% of total capacity
|Deep well injection||Brine is injected into porous subsurface rock formations||13|
|Land application||Brine is used for irrigation of salt-tolerant crops and grasses||8|
|Evaporation ponds||Brine is allowed to evaporate in ponds while the remaining salts accumulate in the base of the pond||4|
|Sewer discharge||Discharge of brine into an existing sewage collection system. Low in cost and energy||27|
|Seawater discharge; Surface||Brine is discharged on the surface of seawater. The most common method for all big desalination facilities worldwide||45|
|Seawater discharge; Submerged||Brine is discharged off shore through multiport diffusers installed on the bottom of the sea|
Fig.3, Most common brine disposal methods in the US
Surface water discharge is the most common alternative because it can be applied to all desalination plant sizes. Sewer disposal is the mostly applied method for the discharges of small desalination plants. Deep well injection application is most suitable for medium and large-size inland BW plants. Land application and evaporation ponds are usually applied for small and medium-size plants where the climate and soil conditions provide for high evaporation rates and year-round growth and harvesting of halophytic vegetation.
The main advantages and disadvantages of the most common brine management options are presented in Table 4.
Table 4, Comparison of Brine Management Methods
Brine management method
Surface water discharge
| || |
| || |
Deep well injection
| || |
| || |
| || |
Table 6, presents the construction costs for 40,000 m3/day BWRO and SWRO desalination plants at 80% recovery - 10,000 m3/d brine and 45% recovery - 48,900 m3/d brine respectively.
Table 6, Construction costs for brine disposal methods of theoretical 40,000 m3/ day desalination plant
Brine management method
BWRO ($ mm)
SWRO ($ mm)
Surface water discharge
Deep well injection
Fig.4, Graphical representation of a rough approach to CAPEX of the conventional brine treatment disposal depending on the brine flow rates
Typically brine discharge to the sewer (limited to small brine flows) or to surface waters (sea, ocean, or river) are entailed better in legislations due to their common use. Lined evaporation ponds with a leakage monitoring system usually are easier to get a permit rather than land application (RIB disposal and spray irrigation) because it is more protective of local aquifers.
The duration of construction of some brine disposal systems, like for example long ocean outfalls with complex diffuser structures, is often the same to the construction time of the desalination plant itself and involves prolonged environmental studies and regulatory review. Also the RIBs and deep injection wells involve detailed and often six-month to one-year-long studies of site suitability and constraints. Discharge to a sanitary sewer is usually the easiest way to implement a concentrate management alternative.
The smallest site typically belongs to sewer discharge and evaporation ponds usually have the largest site requirements.
Deep injection wells are not suitable in seismic zones and require the availability of deep and high-saline-confined aquifers. The injection wells will need periodical inspection and maintenance, which requires either a backup disposal alternative or installation of backup wells.
Shallow beach wells are not suitable when their location has high beach erosion.
Brine management options like evaporation ponds or land application may be seasonal in nature, and in this case a backup alternative is needed to improve their reliability.