Dear Pramod, Depending on ...

Dear Pramod,

Depending on the loading rate of TDS and volumes to be treated, you may either prefer a (sand) filtration system with automated backwashing (fairly cheap) or a percolation wetland system. If the loading rates are very high to start with, you may prefer to start with a primary settling tank to already clarify to a certain extent. The use of coagulants/flocculants will increase TDS removal yet at increased cost.

Following are the cheap method for removing TDS
1.  Biochar

2. Advanced biological treatment

3. Sand filtration with automatic backwash

4. RO (Costly)

Knowing that TDS = total dissolved solids, the first step is to remove all non-mineral dissolved solids by advanced biological treatment (without adding any chemicals other than nutrients if/as required). This results in ultra-low TOC, COD, BOD, TSS, turbidity (NTU) and SDI (silt density index) which minimizes irreversible fouling and power consumption of NF/RO membranes and doubles their economic lifetime. In the second step, some of the minerals could be removed by precipitation and/or air stripping. Finally after UF/cartridge pre-filtration and conditioning, the remaining TDS is retained by partial or full RO filtration as needed. I would like to add that some minerals such as sulfates, nitrates, ammonia, ... can also be removed in the first step by advanced biological treatment. In this way, we develop, design and realize tailored high grade water reuse systems with over 90% overall water recovery = less than 10% water loss at the lowest operating and lifecycle cost. Depending on the scale/capacity and on the quality of the source water the treatment steps 1 and 2 could be skipped as to minimize investment cost.

RO is the only commonly used domestic filtration system that removes even the dissolved impurities. RO is required if the Total Dissolved Solids (TDS) exceeds a certain value. (what is the upper limit ? Look for discussion on that elsewhere in IWP). RO is also suggested if you have reasons to believe that your water may be contaminated with sewage/ pesticides/ heavy metals/ industrial effluents.

A problem with RO is, it needs a lot of water. It divides the input water in two parts, and forces the dissolved solids out from one part in to other. Thus, the output comprises two streams of water – a “clean” stream with low TDS and cleaned of other impurities too. And a “reject” stream that is even more dirty than the input water. Typically, an input of 3 liters will give 1 liter of clean water and 2 liters of “reject”. Theoretically, the “reject” water can be used for mopping the floor etc. but few have the discipline to do that.

regards,

Prem baboo