Case Study: Iron Removal Greensand Filters

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

Process water for a manufactured gas plant was sourced from a local well high in dissolved iron and manganese. This facility uses precision equipment which requires high-quality water free from suspended solids and dissolved solids to operate at peak capacity. Dissolved iron and manganese commonly cause issues with manufacturing processes because they form suspended solids as they’re exposed to changes in temperature, pressure, oxygen content, and more. 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 will lead to damage and system downtime.

There are multiple ways to handle dissolved iron and manganese in water. Overall, this design problem requires a combination of oxidation to form suspended solids and some method to capture these precipitated solids. High-volume air delivery is one approach and chemical injection of an oxidizing agent is another approach. The best fit for this manufacturing facility was determined to be a greensand media filter based on the combination of high 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 specialty filter media provides strong Fe & Mn removal.

Multiple media options exist for an iron removal water treatment system, but they all are manufactured with an oxidant coating to react with dissolved iron and manganese. When dissolved manganese or iron contact the surface of an oxidizing media, it reacts and forms suspended solids which can be filtered out of the water process. A manganese dioxide coated media is the most common solution for dissolved iron filtration and 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 high-concentration dissolved iron 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 which must 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 was accomplished with a 4% potassium permanganate (KMnO₄) solution injected directly into the filtration system influent. 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; able to react with and filter the iron and manganese.

Greensand and other multimedia filters require routine backwash cycles to keep the system operating properly. Media filter backwash cycles are generally backwash based on increasing differential pressure or timers. This system automated backwashes to begin when the differential pressure across the vessels reaches 15 psi. Plant water demands were continuous filtered water output of 650 GPM from our system, which required some custom process management. 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 backwash water was required to meet demands for this project. The supply well was constructed with multiple pumps to account for increased water demand during backwash cycles. The main pump always produces 650 GPM and a second pump kicks on 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. After all four vessels have been backwashed, the filters return to forward flow and the secondary pump is turned off until the next backwashing cycle. Backwash water is directed to a large cone-bottom settling tank for sludge settling and water decant. Filtered solids form concentrated sludge at the bottom of the tanks and can be collected in drums for storage and disposal.