How to Treat Wastewater?
An example of a wastewater treatment plant is below:
A) Pre-treatment (Solid-Water Separation)
There are plenty types of screening, namely the drum screens, basket screens, static screens, and rack screens. If your effluent has significant oil and grease content, you should also use oil and grease traps, because oil and grease will stick onto the pipes and machines.
B) Chemical Treatment
You will start with a pH adjustment step, followed by coagulation and flocculation. pH adjustment is important because only at certain pH range, coagulant works well and some heavy metals such as Zinc and Copper will precipitate.
Depending on your incoming effluent’s pH, pH adjustment chemicals can be acidic (sulphuric acid, hydrochloric acid etc) or alkaline (caustic Soda, lime, soda ash).
Coagulation neutralizes the negative surface charge of the colloidal particles so that these particles would come and stick together. Coagulants are chemicals having positive charges (cationic nature) such as aluminum sulfide (ALUM), Polyaluminium chloride, Ferric Chloride etc.
Flocculation means the formation of bigger particles. When particles are bigger and hence heavier, settling of these to the bottom would be faster and easier. Common flocculants are polymer.
C) Biological Treatment
Biological treatment is needed mostly by effluent samples with high BOD value, for example, those taken from food processing factories or animal slaughtering houses.
Biological treatments can be broadly classified into aerobic and anaerobic processes. In aerobic process, microorganism requires oxygen to break down organic matter, while in anaerobic process, no oxygen is required.
A more commonly used type of aerobic technology is activated sludge, of which inside an aeration tank, there is a constant supply of oxygen to ensure microorganisms grow well in it.
There are also other technologies such as moving bed bio reactor (MBBR), membrane bio reactor (MBR) and fixed bed bio reactor (FBBR). These bioreactors differ in the way the biofilms are placed inside them.
All IETS will generate sludge that needs to be disposed from factories. You will have easier handling and lower disposal costs if you do proper sludge dewatering. There are various technologies to dewater the sludge, for example, by using a filter press, screw press or belt press.
A non-mechanical way to dewater sludge is by using geotubes bags, of which wet sludge is pumped and compacted into these bags, hence water is squeezed out on its own.
The final stage before treated water leaves the IETS is the filtration stage. Typically, both sand and activated carbon filters are used. In the case of any leakage in these filters, bag filters are further used, otherwise, the treated water storage tanks will be contaminated.
Sand filtration can filter suspended solids of size more than 100micron, while activated carbon filter will adsorb COD, odour, and colour.
In some cases where treated water is to be re-used, you can choose ultrafiltration, i.e. filtration using a membrane. It is capable to filter suspended solids of size up to 0.0001micron.
Some IETS will also add in softener to the treated water to remove hardness such as Calcium and Magnesium. Heavy metal removal is also used.