Domestic water purification is very different from industrial filtration. Part of the divergence is due to the type of contaminant. Domestic water may have germs that make it unsuitable for use inside the house so it can be boiled or treated with a domestic purification fluid or using softeners. It can also be manually filtered or disinfected with chlorine.
Other potential contaminants for domestic water include fertilisers or pesticides in a farming area, or sewer waste if the infrastructure is somehow damaged. However, in industrial settings, contaminants are more toxic. They may include chemicals, oil products, metals, glass, and manufacturing by-products.
Usually, industrial wastewater isn’t intended for household use. It’s treated so that it can either be channelled back into the manufacturing process or disposed of safely without damaging the environment. The scale is far larger than domestic filtration, so the methods used require the capacity to handle massive volumes, sometimes hundreds of litres.
Generally, purifying wastewater from a factory is a multistage process. It uses a combination of filtration media, from plastic funnels and fabric membranes to centrifuges and magnets. Your selection of purification will depend on the toxin you’d like to extract, space and time available, and what you plan to do with the water once you’re done.
While physical filtration may seem like an impractical option, it can still be used in certain situations. It functions using gravity, with no electrical or manual force pushing the water into place. This system could consist of a massive drum filled with filtration materials like sand, anthracite coal, or garnet. It’s basically a home filter on a larger scale.
The filtration media is a solid layer rather than a membrane, so the purified water can’t flow back upwards. Gravity prevents this as well. Another variant is RO (reverse osmosis) where the dirty water is driven and purified by differences in pressure.
There are five main types of chemical water purification
- Reduction / oxidation
- Natural zeolites
- Sulphide precipitation
- Hydroxide precipitation
- Ion exchange
Each process uses a different chemical base. Hydroxide precipitation uses lime in its various forms, which include pebble lime, hydrated lime, caustic soda, magnesium hydroxide, and soda ash. An acid may be added after precipitation to restore a safe pH. For the sulphide precipitation variant, sodium hydrosulphide or sodium sulphide is used.
Oxidation/reduction can be used to neutralise arsenic by adding permanganate, sodium hypochlorite, chlorine, ozone, or hydrogen peroxide. To eliminate selenium and chromium, use sodium metabisulphite or sodium bisulphite. As a final option, a resin containing ions of chloride or sodium can be exchanged with the foreign matter you’d like to dispose of. It works well for getting rid of nitrate, arsenic, and other dissolved metals.
The most popular industrial method is to use a filtration machine. One example is the Hoffman Flatbed Vacumatic Filter. It’s useful in construction, mining, excavation, and similar industries. It dries solid waste into a solid cake so that it’s easier and cheaper to get rid of, while the clear water comes out of the other side, ready for re-use or disposal.
Wastewater is pumped onto a conveyor belt lined with a filtration membrane supported on a wire mesh. Air is passed over the dirty water, pushing aside any surface dirt or foam. Beneath the belt, a centrifugal blower or a Hoffman exhauster creates a vacuum.
The vacuum draws water through while increasingly solid waste flows along the belt towards a solid waste collection bin. The filtered water is then drawn into a storage compartment for clean water. The whole machine is automatic, so when the filtration media reaches a certain pressure, a fresh length of the membrane is fed onto the belt and pre-coated to work better.
There are three main areas of biological treatment. They are referred to as suspended growth, attached growth, and bioreacting membranes. Attached growth involves the creation of a biofilm on top of your regular filtration surface made of rock, activated carbon, or plastic. These bacteria then consume any contaminants they come into contact with.
In suspended growth, a biological filtration product is introduced to your wastewater. It catalyses the contaminants to cluster together in biological blocks of similar matter. Suspended growth works particularly well for removing nitrates and ammonia. Activated sludge is a typical example.
Whichever treatment option you select, check your local wastewater regulations. Once your water is cleaned, you’ll still have to get rid of it, and it would be a pity to go through all the trouble of purifying your water only to be fined for liquid littering.