A: Wastewater treatment plants in the upper Midwest, especially those utilizing biological nutrient removal, are susceptible to proliferation of Microthrix parvicella in the winter. Microthrix is classified as low F/M filamentous bacteria; it competes successfully in alternating aerobic/anoxic conditions, thus biological “selectors” actually encourage Microthrix growth. Microthrix has a relatively large growth rate at low temperatures, giving it an advantage over floc formers during the winter.  Given its hydrophobic nature, Microthrix has a tendency to accumulate in floating scum/foam on aeration basins, clarifiers, and digesters. Digesters with gas mixing systems tend to concentrate the Microthrix in thick floating scum layers.

Attempts to control Microthrix via RAS chlorination have generally been unsuccessful. The one sure way to reduce and control Microthrix populations is through consistent, aggressive sludge wasting. Most operators are reluctant to reduce sludge age in the winter, fearing loss of nitrifiers, so there is a bit of a balancing act in reducing Microthrix while maintaining a viable nitrifying population (if required).

Recent research has found that polyaluminum chloride (PAX or PAX-14) can be effective in controlling Microthrix. The PAX dosing appears to attack the ability of Microthrix to use lipids, making the filaments relatively uncompetitive. Although still somewhat experimental, PAX has been successfully implemented in full-scale plants.

 

 

 A:  Vivianite is a hard, dense blue-gray precipitate of hydrated ferrous phosphate (Fe3(PO4)2•8H2O). The dewatering anaerobically digested sludge (combined primary and waste activated) often results in scale formation caused by the precipitating of vivianite or struvite (magnesium ammonium phosphate). The release of biologically retained phosphorus under the anaerobic conditions in the digester causes the digesting sludge to reach a saturated condition with respect to phosphate. Exposure to air and turbulence during sludge dewatering strips out carbon dioxide, resulting in a slight rise in the pH, which is enough to lead to the phosphate precipitation. Typically, the best strategy to diminish the scale nuisance is to reduce the phosphate concentration in the digester by a controlled iron addition to the digester feed (or recirculation) immediately upstream of the digester if possible. Caution is required because iron salts can impact the alkalinity balance in a digester.

 

A: The effectiveness of iron removal of phosphorus in WWTPs is determined by the availability of the iron, which is influenced by mass loading (chemical dosage), dissolved iron state, competing reactions, and efficiency of contact. Theoretically, the ratio of iron to phosphorus is 1:1 (on a molar basis), but the environmental conditions in a WWTP reduce the reaction efficiency. For example, to reduce the effluent phosphorus to below 1 mg/L, an over-dosage of iron must be employed, equivalent to 3 parts iron to 1 part phosphorus (on a molar basis). The iron contained in the backwash of an iron filter will be in the form of iron oxide, not readily available for reacting with the phosphorus in the WWTP. Generally, the quantity and form of iron in filter backwash is such that it will not significantly impact the dosage of iron needed at the plant for phosphorus removal, but adding the backwash waste will help reduce iron demand.

A: Struvite (magnesium ammonium phosphate) forms in municipal anaerobic digesters where the water has a magnesium hardness component. It has a tendency to form in the tanks, pumps, piping systems, and heat exchangers in contact with the anaerobic sludge, especially at valves and elbows where there is turbulence. Struvite forms as a hard, dense crystalline mass and can be a nuisance. It is typically more prevalent in plants utilizing biological phosphorus removal. Phosphorus taken up biologically in the aeration tanks is released in the anaerobic conditions of the digesters, resulting in struvite formation. Recently, there has been a growing interest in harvesting struvite from WWTPs. This interest is driven by the desire to reduce operating problems with struvite, reduce the amount of phosphorus in biosolids that are land applied, and produce a revenue stream from the struvite as a fertilizer.