Explaining Water Treatment
The first step in establishing a wastewater treatment process is to identify the primary reasons for treatment. Why is treatment required?
The key driver may be high-level process optimisations driven by upper management; or specific issues requiring immediate remedy, as flagged by frontline operators and engineers.
Common reasons for industrial wastewater treatment are:
1. High wastewater disposal costs
2. Changing government/EPA (Environmental Protection Agency) regulations
3. Need to reduce or supplement freshwater consumption
4. Ineffective wastewater treatment process in use:
Unsatisfactory treated water quality
Low water recovery (too much concentrated brine produced)
Too many reagents and/or too much power consumption
Equipment prone to breakdowns/fouling
5. Environmental sustainability improvement
Once the primary reasons for wastewater treatment are established, specific treatment requirements can be identified. These are usually unique to each application. The following questions can help identify specific wastewater treatment requirements:
What treated water quality is required? Does the broad contaminant load require treating, or does the wastewater have specific contaminants which require targeting?
Is a zero-brine water treatment process required? If not, what strategy will be deployed for handling a concentrated brine stream?
What is the budget allocation for wastewater treatment – initial capital investment and ongoing operating costs?
Are there already resources on-site that can be used for a wastewater treatment process (reagents, power, steam, etc.)?
Types of Treatment Technologies
A range of technologies are used for wastewater treatment, with most systems using a combination to achieve the required treatment outcomes.
Screens and Filters
Screens and filter systems provide a physical barrier that traps macro contaminants while still allowing the wastewater to pass. Screens remove large items (greater than 1mm), with the cut-off size determined by the aperture of the screen – a larger aperture screen will target larger items. Filter systems are used for small particles suspended in the wastewater (less than 1mm). Many types of filters are used in the water treatment industry, however the most common are cartridge filters – which use replaceable cartridges to remove the suspended particles – and media filters, which use a bed of media (such as sand).
Membrane systems are long tubular vessels which contain semi-permeable membranes. These membranes act as molecular barriers that target and remove chemical contaminants based on molecule size and charge. Wastewater is pumped through the membrane system at high pressure, with certain molecules (such as water – H2O) permeating through the barrier to become treated water. The type of membrane selected determines the level of treatment.
The following are three common membrane types used for water treatment:
Ultrafiltration (UF) membranes – these contain larger pores, which reject macro-molecules such as proteins and hydrocarbon chains
Nanofiltration (NF) membranes – these contain medium-sized pores, which reject macro-molecules and divalent ions such as sulphate (SO4) and metals (Ca, Mg, Cu, Fe)
Reverse Osmosis (RO) membranes – these contain the smallest pores, which reject nearly all dissolved molecules (including sodium and chloride) while still allowing water to pass through
A disadvantage of the small membrane pores is their tendency to foul, restricting the ability of the membrane to treat the water. As such, membrane systems are usually combined with up-front filters and membrane cleaning equipment (CIP), which improve system durability and operability.
Ion exchange systems are essentially vessels containing chemically activated resin beds. As wastewater passes through the resin bed, chemical contaminants are targeted and exchanged for innocuous ions. There are two common types of resins – cationic and anionic – which remove positively and negatively charged molecules respectively from the water.
Once the resin bed has exhausted its ion exchanging capacity, it is regenerated using dilute chemical reagents such as sulphuric or caustic acid. The resin is then washed and ready to continue treating the water.
Precipitation is a chemical phenomenon, whereby a change in water conditions (pH, temperature, composition) can cause certain soluble contaminants to become spontaneously insoluble. There are many types of precipitation reactions used in wastewater treatment; and selecting the correct one depends on the chemical composition of water and the type of contaminant targeted for removal.
Lime precipitation is commonly used, particularly by the mining industry, due to its simplicity and effectiveness at treating a wide range of chemical contaminants. Limestone (calcium carbonate) or hydrated lime (calcium hydroxide) is added to the wastewater, causing the pH of the water to rise. This promotes the precipitation of metal hydroxides and gypsum, converting these components (sulphate, hardness, metals, acidity) from the soluble liquid phase into a solid. Once the precipitation reaction has occurred, the water mixture is passed through a clarifier to extract the solid materials, creating a low-volume sludge by-product.
Bioreactors use microbiology to treat nutrient-based contaminants (nitrates, ammonia, phosphates, COD). The type of water treatment depends on the biological species selected, with conditions manipulated in the reactor to promote colony growth and productivity. Common types of bioreactors include moving bed (MBBR) – where biology is housed in a porous substrate that moves around the reactor – and membrane bioreactors (MBR), where the biology is fixed to a membrane that removes contaminants as the water passes through.