What is ZLD?
ZLD stand for Zero Liquid Discharge Plant. As the name states it removes all the liquid waste from the effluent. It includes pre-treatment of effluent with evaporation due to which precipitation occurs. After precipitation crystal are formed. During the process of evaporation water vapors form which condensate & are returned to the initial process. It helps to meet higher recovery rate & to recover valuable materials from wastewater.
ZLD is divided into two major parts
Process of ZLD occurs mainly in two parts i.e.
- Pre-Concentration – It occurs either by electrodialysis or membrane brine concentrators & helps to recover almost 65-80 % of water.
- Evaporation – After electrodialysis or membrane brine concentrators next step process is achieved through evaporators. It evaporates remaining water and then vapors are condensate for reuse of waste. The left remaining goes for precipitation it boils water until and unless all impurities are crystalized.
Process flow of ZLD
Stages of ZLD
- Pre-Treatment Stage – It is the first initial & important stage. It includes Lime-Soda Ash Softening, Electro Chemical Treatment & Advance Biological Treatment like MBR or MBBR.
- Recycling Stage of Water – To reduce high TDS of water multistage RO system are used. Sea Membrane works at high TDS & it requires a lot of maintenance as due to fouling & feed of stringent water. To get water recovery more than 90%, EDR can also be used.
- ZLD Evaporation Stage- After water is filtered through pre-treatment & recycling stage, then is goes into evaporator. The most commonly used evaporators used are MEE (Multi Effect Evaporator) & MVR (Mechanical Vapor Recompression). For higher wastewater quantity MVR are used.
Evaporation Technologies on ZLD
- ATFD- Agitated Thin Film Dryer– It has adjustable blade arrangements which gives flexibility to different kinds of crystallizable salts in less than 10 % of moisture. It is placed on the same platform as that of evaporator flash vessel.
- Forced Film Evaporators- It has reverse feed arrangement which gives maximum utility of motive steam. It is Ver cost effective solution.
- Hybrid Falling Film & Forced Circulation Evaporators- It is a solution for low salinity between 3-10 %.