The Treatment of Wastewater for re-use in Industry

Test and Measurement conference October 2015

JA Reeves

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Presentation Outline

• Introduction

• Industrial processes for water treatment

• Ion Exchange Resin

• Reverse Osmosis

• Biological Processes

• Evaporation and Crystallisation

• Conclusion

Introduction

• Single largest water user

• Zero Liquid Effluent Discharge (ZLED)

• Concerns about re-use of effluents

• Deteriorating incoming raw water quality

• No single solution

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Ion Exchange Resin

• Used in the following plants:

• Demineralisation (Demin) Plant

• Condensate Polishing Plant (CPP)

• Demin Plant

• Most, particularly newer plants have 4 vessels (cation, mixed bed, strong base and weak base).

• CPP

• Used to treat the steam cycle water in case of a condenser tube leak

• The quality of water is carefully controlled and maintained

• Stringent chemical limits on the steam cycle

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Ion Exchange resin (Contd)

• Conditioning of the resins

• Cation Exchange: Hydrogen Form : Hydrochloric Acid

• Anion Exchange: Hydroxide Form: Sodium Hydroxide

• Test and Measurement Requirements

• Quality of water produced

• Condition of the resins

• Physical Tests carried out on the resin;

• osmotic shock, number of cracked and broken beads, etc

• Two Important Resin Tests:

• Capacity and kinetics 5

Ion Exchange resin (Contd)

• Large volumes of waste water are produced in the treatment and analysis of the resin.

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Reverse Osmosis

• The process

• A saline solution is placed on one side of a tank and pure water on the other side, separated by a semi-permeable membrane.

• Sufficient pressure (approximately 50 to 60 bars) is applied to the saltwater side of the tank to counteract the natural osmotic pressure from the pure water side, and then to push the saltwater through the filter.

• Because of the size of the salt molecules, only the smaller water molecules make it to the other side, thus adding fresh water to the water side, and leaving the salt on the other.

• Test and Measurement Requirements:

• Product water tested: Potable water standards

• Waste produced

• complex mixture of salt 7

Reverse Osmosis (Contd)

• RO Schematic

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Introduction to Biological Processes

• Advantages

• biological processes are allowed to continue naturally with little to no human involvement

• Disadvantage

• Slow compared to other processes

• Processes to be discussed:

• Biofilm remediation of contaminated water

• Utilisation of biofilm for the reduction of organics

• Wetlands

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Biofilm Remediation of Contaminated Water

• Bioremediation of heavy metal contaminated water is a proven passive technology

• Bacteria are found everywhere

• Some extremophiles are able to survive high pH, temperature, pressure, salinity and toxicity.

• Bacteria present in low quantities: environment can be adjusted

• Bacterial biofilms of the native population are allowed to grow on a fixed substrate matrix (stones), under controlled conditions and the contaminated water passed over it.

• This technique has been proven able to remove hexavalent chromium, magnesium, sulphate, chloride and nitrates, without the formation of a waste product for removal/treatment.

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Practical Application of Biofilm Process

• Ash Dump

• Hexavalent Chromium

• Flow from the ash dump is estimated as 172.8 m3/day

• commercially proven on flows of up to 300 000 m3/day

• Mine Water

• Decanting Acid Mine drainage

• Modular reactors

• All water treated will be re-used

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Utilisation of Biofilm for the Reduction of Organics

• Process

• Water is passed over a biofilm in a bioreactor

• The bacteria in the biofilm utilise the organics in the water as nutrients

• Biofilms removed, centrifuged out of the water

• Sidestream process

• Reduction in raw water intake

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Wetlands

• Constructed/artificial wetlands: engineered systems which utilise natural functions of vegetation, soil and organisms to treat the water stream

• Mimic the features of natural wetlands: biofiltration, sediment removal, removal of pollutants, may serve as a habitat for wildlife. Results in improved water quality without impacting environment

• Improvements are:

• Reduction in suspended solids

• Dissolved biodegradable material is decomposed by microorganisms to carbon dioxide and water

• Nutrient removal

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Evaporation and Crystalisation Techniques

• Evaporation Techniques:

• Evaporation Ponds

• Natural evaporation from lined ponds

• The remaining salt is either left in the pond or removed and disposed of

• Evaporation ponds are highly dependent on the evaporation rate

• Evaporation rate decreases exponentially with increasing salinity

• Tests required to determine the rate are: concentration of brine (TDS), composition of brine (type of salts present), salt precipitates and rate of disposal.

• The following environmental conditions should also be known: temperature, humidity, solar radiation and wind speed

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Evaporation Pond

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Wind Aided Intensified eVaporition (WAIV)

• 15-20 times more evaporation intensive than traditional evaporation ponds.

• a wet surface made from a specific adsorbent material is suspended vertically from constructed modules

• A pump transports brine to a distribution network from which the strips of adsorbent material are gravity fed.

• Wind is used to drive away excess humidity to improve evaporation rates

• Evaporation rates are calculated from vapour pressure of the air, vapour pressure of evaporating water body and wind speed

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Wind Aided Intensified eVaporition (WAIV)

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Forced Evaporation – Water Cannon

• Brine is pumped from the pond to the water cannon at a high pressure and then sprayed back into the pond over a large area to increase the evaporation rate

• Factors affecting the evaporation rate are: air temperature, relative humidity, wind speed, water droplet size and spray coverage area.

• Major disadvantage: Salt drift can be a cause for environmental concern and needs to be controlled

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Vacuum Multi Effect Membrane Distillation (V-MEMD)

• Operates at low pressures and temperatures.

• Concentrate passes through various stages

• Becomes increasingly concentrated as clean water is extracted at every stage

• The system operates just below the crystallization point

• Membrane ensures that vapour can pass but water cannot

• Distillate product is pure water

• Concentrate contains all of the contaminants

• The following measurements are critical: temperature, pressure, brine and product water analysis. Temperature and pressure must be strictly controlled

•

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V-MEMD Schematic

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Membrane Distillation and Crystallisation

• A brine stream is treated and 100% water recovery can be achieved.

• Much like V-MEMD but concentrating the mother liquor to such an extent that nucleation and crystal growth can occur under a controlled environment.

• Temperature and pressure controlled

• Clean distillate product stream and a pure solid salt product

• Brine ion, product water and dry salt analyses are required

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Membrane Distillation and Crystallisation Schematic

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Freeze Crystallisation Techniques

• A brine stream is cooled to separate it into ice and salt

• Ice is melted to provide a pure water stream for re-use

• Salt can be used for re-sale (depending on its purity) or disposal

• Energy utilisation is lower than that for evaporation techniques

• Heat of fusion of ice is six times less that of the heat of evaporation of water

• HybridICE

• This technology feeds brine into a HybridICE generator (HIG) which cools the brine stream until it reaches its first-ice point

• An ice slurry results and the ice is separated from the concentrated brine solution in a HybridICE filter (HIF)

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Freeze Crystallisation Techniques (Contd)

• Product ice used to pre-cool the brine feed into the HIG thus optimizing process energy

• Filtered brine concentrate is sent to the vacuum evaporator (VAE) where it is continuously circulated through a heat pump

• Measurements for this process include temperature, pressure, brine feed analysis, product water analysis and dry salt analysis

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Freeze Crystallisation Techniques (Contd)

• Eutectic Freeze Crystallisation

• A solution containing dissolved contaminants (ie. brine) is frozen slowly until water ice crystals form and float to the surface.

• Contaminants concentrated in the remaining solution

• Ice is scraped off and removed

• The ice crystals can be washed and melted to yield a pure water solution

• Salts can be crystallised out of solution at their eutectic temperature

• The high purity salts can potentially be packaged for resale or re-use in other processes

• Brine analysis is required in order to thermodynamically model the multi-component system to determine the temperature operating points of the system

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Conclusion

• All technologies described have their uses.

• Ion exchange resin treatment is very effective in obtaining high purity water, but produces large volumes of waste.

• This waste can be sent for biological, evaporation or crystallization technologies

• Analysis of the brine stream is critical for the effective design of evaporation and crystallization processes.

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Thank you Acknowlegements

Kelley Reynolds-Clausen

Kelly Whitehead

Eskom

NLA