by Bill Ericson

Phosphorus has taken center stage in recent years, primarily from the standpoint of tighter environmental regulations. Another view of phosphorus has also been emerging, that it plays a significant role in worldwide agriculture, and that it is a valuable finite resource. Current estimates indicate that known phosphate reserves will be depleted in approximately a century. Given that phosphorus will become increasingly scarce (and more expensive), alternative, more sustainable sources of phosphorus are under consideration. One of those sources is wastewater. When extracted from wastewater treatment processes, the recovered phosphorus improves the sustainability of fertilizer supply and food production. Wastewater management organizations are increasingly taking steps to capture, and benefit from, their phosphorus.

The Madison Metropolitan Sewerage District (MMSD) is one such organization. MMSD is taking an innovative approach to reducing the maintenance challenges and costs associated with control of phosphate-based inorganic chemical precipitates at its Nine Springs Wastewater Treatment Plant (NSWWTP). The 50 mgd NSWWTP operates with an enhanced biological phosphorus removal (EBPR) process, where soluble phosphorus is removed from the bulk liquid and stored as intracellular polyphosphate in phosphorus accumulating organisms (PAOs). Waste activated sludge (WAS) from an EBPR process is enriched with high phosphorus concentrations that are further increased after sludge thickening. Secondary phosphorus release occurs when the sludge is fed to the plant’s anaerobic digesters.

Formation of struvite (magnesium ammonium phosphate) is a common problem in anaerobic digesters and the downstream dewatering equipment. Struvite crystals create scaling in pipelines, walls, and process equipment, resulting in reduced capacity, as well as operation and maintenance problems. The NSWWTP has experienced struvite scaling in draft tube mixers, heat exchangers, heat recirculation pumps, and sludge transfer lines. A considerable fraction of the phosphate removed in the EBPR process is recycled back to the EBPR system when gravity belt thickening (GBT) filtrate is recycled to the headworks. Sidestream treatment of the GBT filtrate is employed to reduce the phosphorus recycle, utilizing ferric chloride dosing.

Generally, the District employs treatment of GBT filtrate, feeding iron in a mole ratio of 1.5 times the P content in the filtrate. Variability in the consumption of iron increased significantly after the NSWWTP’s 10th Addition to deal with elevated levels of struvite scaling that occurred with thermophilic digestion. MMSD’s experience is that reliance on iron salts for struvite mitigation is expensive and only partially effective.

With hopes of decreasing its dependence on iron salts, MMSD completed a Solids Handling Facilities Plan that outlined a plan for the biosolids treatment and handling at the NSWWTP. The 20-year plan recommended that an advanced digestion process, multi-stage acid phase digestion, be implemented through the next major construction project, the 11th Addition to the NSWWTP. One of the improvements recommended in the Solids Handling Facilities Plan was that the struvite harvesting process be implemented as a means of mitigating the uncontrolled formation of struvite scale in the solids handling system.

In the struvite harvesting process, phosphorus is recovered from the treatment processes in a controlled manner, in a pellet form suitable for the fertilizer market. Struvite harvesting was chosen over ferric chloride feed because of substantial operation and maintenance savings. Evaluation of struvite harvesting revealed that implementation at the NSWWTP would be cost-effective due to resulting reductions in the use of iron salts, chemical sludge production, and aeration power demands. Over the 20-year period, total annual operation and maintenance savings will average $1.5 million per year, yielding a simple payback of eight years. Struvite harvesting will also lower the phosphorus content of the Metrogro biosolids product and will create a revenue stream from the pelletized fertilizer sales.

Following onsite pilot testing of the technology, MMSD selected the struvite harvesting system developed by Ostara, the Pearl® Nutrient Recovery System, which yields the Crystal Green® fertilizer product. The Ostara process uses a proprietary fluidized bed upflow reactor to recover about 85% of the phosphorus throughput. At the NSWWTP, the struvite harvesting system will be recovering phosphorus in filtrates from gravity belt thickening of WAS and digested sludge. Phosphorus recovery from the WAS will be maximized through a pretreatment step to promote phosphorus release prior to thickening. Recovery of phosphorus prior to and following anaerobic digestion will substantially reduce the uncontrolled formation of struvite in the solids handling system, as well as drastically reduce the demand for ferric chloride. At design conditions, the struvite harvesting system will be treating 1.2 mgd of combined filtrates, recovering 1,400 lbs/day of phosphorus, and yielding 2,100 tons/year of fertilizer product.

Implementation of struvite harvesting at the NSWWTP will be accomplished in MMSD’s next major plant expansion, anticipated in 2011-2012. The installation will be Ostara’s fifth commercial-scale system operating in North America and the first in Wisconsin. MMSD will own and operate the struvite harvesting system, while Ostara will be responsible for marketing, sale, and delivery of the Crystal Green® fertilizer product. The Crystal Green® product is a slow-release fertilizer with nutrient content of 5% nitrogen, 28% phosphorus, and 10% magnesium. The process yields highly pure, hard crystalline pellets of uniform size, suitable for fertilizer blending and application.  

For MMSD, struvite harvesting holds the dual promise of lower plant operating costs and beneficial recovery and use of an increasingly valuable resource–phosphorus.