Iron-sand filter bed clogging poses the greatest operational and maintenance challenge of all filters. Poor drainage of the filter due to clogged filter surface, inlet, outlet, and underdrains, and persistent tailwater has the potential to cause the iron to foul and move within or be lost from the filter bed. When the iron is completely consumed after some time of operation, the entire iron-sand filter bed will need to be replaced. The operating life span has been estimated using iron-enhanced sand columns for phosphate retention to be 35 years (Erickson, et al. 2012). However, this has not been verified in the field. Currently the operating lifespan of the iron material is not known and other indicators will need to be used to judge the filter bed condition such as reduced phosphorus removal performance or a reduction in the clarity of the stormwater treated by the iron-sand filter bed (see discussion below).
Implicit in the design guidance in the previous section is the fact that many design elements of filtering systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
Proper construction methods and sequencing play a significant role in reducing problems with operation and maintenance (O&M). In particular, with iron-enhanced sand filter construction, the most important action for preventing operations and maintenance difficulties is to ensure that the contributing drainage area has been fully stabilized prior to bringing the practice on line (this is a Required practice).
Inspections during construction are needed to ensure the iron-enhanced sand filter is built in accordance with the approved design and standards and specifications. Detailed inspection checklists should be used that include sign-offs by qualified individuals at critical stages of construction, to verify the contractor’s interpretation of the plan is acceptable with the designer. An example construction phase inspection checklist for filtration can be found at this link. This list is also applicable to iron-enhanced sand filters.
Similar to other filtration practices, iron-enhanced sand filters require maintenance. Without regular maintenance, iron-sand filtration media can become clogged and the filter may not be able to convey water. This can lead to stagnant water, iron fouling, and reduction or elimination of pollutant removal capacity.
Effective long-term operation of iron-sand filters necessitates a dedicated and routine maintenance schedule with clear guidelines and schedules. Post-construction considerations for filtration BMPs are applicable to iron-enhanced sand filters.
During the operating life of an iron-enhanced sand filter, phosphorus of several forms and other stormwater constituents will bind to the iron. Currently the operating lifespan of the iron material is not known and other indicators will need to be used to judge the iron-enhanced sand filter bed condition, such as reduced phosphorus removal performance or a reduction in the clarity of the stormwater treated by the iron-enhanced sand filter. If performance declines notably, the iron-enhanced sand media may need to be replaced. Due to iron’s capacity to bind with several stormwater constituents (e.g., fluoride, sulfide, bicarbonate, natural organic matter, and phosphate); analysis of the iron and phosphorus content of the iron-enhanced sand bed can only provide an approximation of the remaining phosphate binding capacity. Total phosphorus at the outlet of the iron-sand filter that consistently exceeds 60 to 70 micrograms per liter may be used as an indicator that the phosphorus binding capacity of the iron-enhanced sand bed has been consumed. If this condition is true, then it is recommended that samples be taken from the iron-sand bed and analyzed for total phosphorus and total iron. Total phosphorus to total iron ratios that exceed 5 milligrams of phosphorus per gram of elemental iron (Erickson et al., 2007, 2012) indicate the phosphorus binding capacity of the iron-sand bed is exhausted and should be replaced.