Basic Flow Path
Ok, lets start with the basics, what does the flow path through an EDI module look like?
The Feed (RO Permeate) is split into two inlet streams, 1) Dilute Feed and 2) Concentrate Feed. There are two outlet streams, 1) Product and 2) Reject.
To obtain the best EDI systems have to offer (consistent and continuous chemical-free operation), proper pretreatment design and EDI system design is a MUST! Shortcuts should not be taken!
EDI system Safeguards - There are several key safeguards that all EDI system should have in place to protect the EDI modules from improper operation. Some are required, some are highly recommended.
- Required safeguard
- Low flow alarm for product and reject flow - shown below as FE for Flow Element and FAL (Flow Alarm Low)
- RO (or EDI feed pump) interlock so EDI “runs” only when RO or feed pump is in operation - Shown on DC power controller as an interlock.
- Optional safeguard
- Product divert valve (Sends low product quality to drain)
- Low feed quality alarm (Protects EDI from high hardness or chlorine/ORP), shown as AE for Analysis Element and AL for Alarm Low
EDI systems that include ion exchange softening can usually operate at 95% recovery.
EDI systems that DO NOT include ion exchange softening must operate at 90% recovery or below.
NOTE: In some cases the recovery may be limited by the concentration of silica in the feed water, where the maximum recovery can be calculated as follows:
- Maximum recovery, % = 100 – (4 x ppm feed silica)
In the field, the silica concentration in the EDI feed water can be measured using a silica test kit.
Suggested Test Kit: Use Silica Test Kit 0.02ppm (20ppb) – Hach Model #2255000
Reclaim of EDI Reject Water
There is normally no problem using the EDI reject water for other purposes elsewhere in the plant as long as it is sent to an atmospherically vented tank to allow venting of hydrogen gas.
There are two potential issues with the recycling the EDI reject stream to the feed of the RO system: carbon dioxide (CO2) and hydrogen (H2).
1) CO2 in EDI Reject Water:
Feed water CO2 will pass through the reverse osmosis system, be removed by the EDI module and transferred to the EDI reject stream. Recycling the EDI reject to the feed of the RO system will typically result in about a 3X increase in the EDI inlet CO2 concentration, which will affect the EDI performance. This must be taken into account when performing the EDI performance projection. The use of degasification or caustic injection pre-RO can prevent the 3-fold increase in CO2 concentration.
H2 in EDI Reject Water:
To prevent the buildup of hydrogen the EDI reject must be returned to an atmospherically vented tank. Do not return the EDI reject directly into a pressurized line.
Summary - EDI Reject Reclaim
- Reject contains electrode gases & CO2
- Returning reject to RO feed increases EDI feed CO2 by up to 3X (if no degasification or NaOH injection before RO)
- May affect EDI product water quality
Other EDI "Watchouts" – Hydraulic
- Water hammer
- Simultaneous operation of product and divert valves
- Should open one before closing other
- Pressure differential between compartments
- Keep pressure slightly higher on product side (2-5 psi recommended)