How to Build a Reverse Osmosis (RO) Treatment System

Reverse Osmosis (RO) is a complex yet highly effective method for purifying water. While building a DIY RO system might seem challenging, it is possible with a thorough understanding of the process and the right equipment. This article will guide you through the steps to build a 1000 lph RO system from scratch.

Preparation and Design

Before starting any project, it's crucial to have a solid design. Let's assume we're building a 1000 lph RO system based on a feed water with a recovery rate of 70%. The permeate recovery rate would be 700 lph, with 300 lph going to concentrate water. With a feed water TDS level of 750 ppm, the concentrate water will have a TDS of approximately 2625 ppm. This suggests we need a seawater membrane type for our design.

TDS and Concentration Factor

Calculating the TDS and concentration factor is crucial. The concentration factor is 3.33, and with a TDS of 750 ppm, the concentrate will have 2625 ppm TDS. This simply means that if the TDS is very high, say 15000 ppm, we may require a seawater membrane type instead of a brackish water type. The recovery rate should be within the permissible range, prompting us to have a lower recovery rate for better performance.

Flux and Membrane Selection

Determining the flux is necessary to select the appropriate membrane. Flux, denoted as the amount of water passing through the membrane surface per day, is deemed important. For our source water, the flux is taken as 14. This flux value helps determine the number of elements required for the system. Most membrane brands have a salt rejection rate of 90-95%, which is crucial for determining future cleaning or replacement needs.

Pre-Treatment Requirements

Our feed water, based on the analysis, contains heavy metals such as lead, arsenic, selenium, and high levels of iron above 7.5 g/L, magnesium/calcium carbonate/hydrogen carbonate, and very hard water. Here’s the step-by-step pre-treatment process to remove these contaminants:

Iron Removal: Aeration system followed by a deep-bed Birm FRP filter. Heavy Metals: Deep-bed activated alumina FRP filter. Hardness: Deep-bed mixed ion exchange/resin softening unit for hardness reduction.

The pre-treatment stages are as follows:

Deep-bed mixed sand/activated carbon 12″ x 52″ FRP unit for particles removal and smell/microbes adsorption (pressure drop 10.3 Bar, Q1200 lph). 12″ x 52″ FRP filter with a deep-bed Birm filter for iron extraction (pressure drop 10.3 Bar, Q1200 lph). Deep-bed activated alumina 12″ x 52″ FRP unit for heavy metals removal (pressure drop 10.3 Bar, Q1200 lph). Deep-bed mixed ion resin softening unit for hardness reduction (pressure drop 10.3 Bar, Q1200 lph).

Pressure Drop Calculation and Pump Sizing

The theoretical total pressure drop in the FRP units is calculated, considering media-induced pressure drop, pipe pressure drop losses, and the total pressure drop for the 6 cartridge systems. This information, along with the flow rate, is used to select the appropriate feed raw water booster pump, ensuring a 20-30% variation due to pressure increases. Proper piping and flow visualization tools, such as Chem-CAD or Aspen, are utilized to ensure the correct process flow for each unit, with non-return valves, manual or pneumatic ball valves, and unions for maintenance.

RO Feed and Antiscalant Dosing

With the pre-treatment complete, the RO feed system is designed to include a pressure gauge to monitor pressure, a possible second secondary booster pump, and a pressure transducer or inline pressure gauge. Antiscalant dosing units and SMBS (Smart MBR Submersion Membrane Bioreactor) are installed to prevent scaling and remove any combined or free chlorine compounds. The dosing pumps are selected based on Q1200 lph and pressure in bars.

High Pressure Pumps and Control Panel

Several high-pressure centrifugal pumps are installed prior to the membrane entrance, featuring a solenoid valve for back pressure prevention. CIP (Chemical Cleaning In Place) provisions are made, with electronic pressure transducers and inline TDS meters installed at both the feed and permeate sides. The control panel features independent displays for pH, TDS/Conductivity, and flowmeters to allow for easy troubleshooting. A mass balance and water analysis confirm the system's efficiency and accuracy.

Conclusion

Building a RO system requires careful planning and execution. From pre-treatment to pressure drop calculation and pump selection, every step is crucial. Understanding the process and having the right equipment are key to success. This process ensures a high-quality, efficient RO system tailored to meet specific water purification needs.