Case Study: Iron Removal Greensand Filters

Challenge: Dissolved Iron and Manganese cause fouling in sensitive process equipment 

A manufactured gas plant sourced process water from wells with high dissolved iron and manganese loading. The sensitive equipment in this facility requires high-quality water, free from dissolved and suspended solids to operate at peak capacity. Dissolved iron and manganese commonly cause issues with manufacturing processes because they precipitate out of solution when exposed to changes in temperature, pressure, and oxygen content. Common issues with precipitated iron & manganese include fouled piping, plugged nozzles, biological slime, and premature media changeouts. This cycle of scale buildup on in process equipment can lead to system downtime and damage.

There are multiple ways to manage dissolved iron and manganese in water. Removing iron & manganese generally requires a combination of oxidation to form suspended solids and physical capture of those precipitated solids. We determined a greensand media filter was the best fit for this manufacturing facility based on the combination of high-volume water flow and dissolved Fe/Mn loading. This greensand system uses a set of (4) 96” diameter ASME vessels with an automated backwash header to deliver 650 GPM of iron-free water throughout the plant.

Solution: Oxidation and filtration with a custom greensand filter provide strong Fe & Mn removal.

Multiple media options exist for an iron removal water treatment system, but they all use an oxidant coating to react with dissolved iron and manganese. A precipitation reaction occurs as water carrying dissolved manganese or iron contacts the surface of an oxidizing media. The suspended solids formed in this reaction can be physically filtered out of the water process. A manganese dioxide coated media is the most common solution for dissolved iron filtration; composition varies across several products. Filter options like Katalox Light and Filox-R are designed to provide iron treatment without continuous chemical amendment. Manganese greensand filter media is designed to work in tandem with chemical regeneration. Greensand filter systems are capable of handling elevated dissolved iron loading with support from potassium permanganate or sodium hypochlorite amendment.

The oxidizing reaction of dissolved iron with manganese dioxide consumes the capacity of greensand filter media, but capacity can be regenerated as the system operates. Iron loading in this project approached 10 mg/l which was high enough to require greensand with continuous media regeneration. This system injected a 4% potassium permanganate (KMnO₄) solution into the filtration system influent to regenerate greensand. A pair of chemical metering pumps precisely doses the process water with KMnO₄ solution from a storage tank. This chemical injection process keeps the greensand bed primed and capable of iron and manganese filtration.

Greensand and other multimedia filters require routine backwash cycles to keep the system operating properly. Media filter backwash cycles are generally based on increasing differential pressure or timers. This system automated backwashes to begin when the differential pressure across the system reaches 15 psi. Plant water demands required continuous 650 GPM filtered water output, this required custom process management in this system. This plant preferred to minimize additional water tanks & storage, so a self-sourced filter backwash design was utilized.

Media filters with self-source backwash directly recycle filtered water from duty vessels to backwash the offline vessel. Water used for backwash is unavailable for the process use, so supplemental water was required to meet backwash demands for this project. The supply well included multiple pumps to account for increased water demand during backwash cycles. The main pump always produces 650 GPM and a second pump activates for an additional 600 GPM backwash flow when required. Both pumps feed the filter system inlet for a combined 1,250 GPM during backwash cycles; which splits 650 GPM delivered to the plant and 600 GPM used for backwash.

Filters are sequentially backwashed to remove filtered iron fines from the greensand media beds. The filters return to forward flow and the secondary pump is turned off after all four vessels have been backwashed. Backwash water is directed to a large cone-bottom tank for sludge settling and water decant. Filtered solids form concentrated sludge at the bottom of the tanks and are routinely collected in drums for storage and disposal.