Since May 2012, I have been involving in an EPC (Engineering, Procurement, and Construction) project. In my opinion, it is a big project because the core business of my company are (but not limited) feasibility study, plant assessment, turn around, plant re-activation, etc.
The first job I need to perform was to create Piping and Instrumentation Diagram of Water Treatment Plant. It was quite challenging task because this is the first time I deal with P&ID and also the first time I deal with Water Treatment Plant.
The first thing you need to do to create the processes required in Water Treatment Plant is you need to make a list of required water, what kind of water you want to produce?
In my case, I designed Water Treatment Plant to produce three kinds of water: (1) Cooling water makeup for cooling tower and process water, (2) Domestic water mainly for sanitary, and (3) Demin water for Boiler Feed Water (BFW). After defining what kind of water I want to produce, I need to understand the characteristics of water source. The characteristics of water will determine what kind of process we need to set up in Water Treatment Plant.
I learn there are several water characteristics that need to be considered:
- Total dissolved solid and total suspended solid
- Total hardness as CaCO3
- Silica and iron
One thing that you also need to consider is the desired water characteristics you want to get. For example, to produce demin water, I need to produce water with conductivity less than 100 microsiemens/cm (conductivity is a measure of how much mineral content is). Conductivity of boiler feed water is one parameter need to be considered as because high concentration of mineral will attack the steam boiler materials and form deposits.
I learnt that water treatment plant usually consists of multimedia filter, reverse osmosis unit and clean in place, and mix bed polisher. If you want to produce domestic water for sanitary, you need to add desinfectant injection (usually using hypochlorite) to the system.
Multimedia filter which consists of sand filter and activated carbon is acting as pre-treatment. The function is to remove suspended solid from water source. Multimedia filter is equipped with backwash system. Multimedia filter is usually installed with pressure difference indicator (PDI), so that when pressure difference accross bed reach specified limit, backwash is required.
Reverse Osmosis is one system in water treatment plant used to produce purified water. Water produced from reverse osmosis unit will have less total dissolved solid and less impurities.
There are several treatment required before feeding water into reverse osmosis unit:
- Alkali dosing. Sometimes water produced from multimedia filter consists of high value CO2 that makes the pH very low. That is the reason why we need to provide alkali dosing
- Anti Scalant dosing. Dissolved minerals are usually concentrated on reject side during the operation of reverse osmosis. This will lead to scaling on membrane surface which will reduce performance of RO system. Anti scalant is used to prevent the issue.
- Sodium Metabisulfite (SMBS) Dosing. Chlorine is very harmful to membrane as it is a strong oxidizing agent. SMBS reacts with chlorine form neutralized salt. It will be disposed by RO membrane during the operation.
- Cartridge filter. Cartridge filter is used to filter very fine suspended matters thus supply highly filtered water.
Mix Bed Polisher
Mix bed polisher is used to reduce conductivity and total suspended solid of water in water treatment plant. It is used as polishing after reverse osmosis unit. Low conductivity water is achieved by exchanging positive and negative ions in water with hydrogen and hydroxide ions.
Like multimedia filter and reverse osmosis unit, mix bed polisher is also need to be regenerated. The cation resin is regenerated with acid and the anion is regenerated with sodium hydroxide.
If you wish to produce domestic water, you need to add desinfectant injection unit into your water treatment plant. It usually consists of chemical tank with mixer (if necessary for dilution), dosing pump, and piping.
I hope this post will give you the idea to design Water Treatment Plant.