Reduce – Reuse – Recycle … Struvite
“Reduce – Reuse – Recycle” has become a common way of thinking. Water industry thinking has taken big steps toward making this slogan a reality in everyday treatment operations. Waste treatment facilities have become resource recovery stations, enabling reuse of water resources. These recovery stations not only prevent organics and excess nutrients from entering our waters, they do it with even greater efficiency, wasting nothing. Increasingly, “resource recovery” means generating energy and byproducts rather than filling landfill space with waste. Treated water is a valuable resource for the community.
For example, the same nitrogen and phosphorus nutrients that can cause harmful algae blooms and eutrophication of our natural waters are employed to propel the biological treatment processes that remove organics from collected waste water. In this biological treatment process, insoluble nitrogen and phosphorus become soluble, and concentrated enough to be transformed into a useful fertilizer called “struvite”. Struvite provides a soluble source of agricultural magnesium, nitrogen, and phosphorus as important as water for the growth of crops and other plants.
Struvite formation is rare, compared to common scaling resulting from calcium treatment agents.
Not every sewerage is suitable as a raw material for struvite.
First, nitrogen and phosphorus must be present in soluble “ammonium” and “phosphate” form, and most waste streams contain most of these nutrients as part of organic matter. Fortunately, the biological treatment process naturally breaks down the nutrients to soluble form through the action of bacteria and other micro-organisms. Magnesium is a micro-nutrient present in both plants and animals.
Second, these three soluble nutrients must be present in just the right concentrations (molar ratio 1:1:1 or weight ratio – see below table) for the chemical reaction to take place. Organically bound magnesium is present in wastewater, and the biological action of bacteria also converts magnesium to its soluble form, where it joins the magnesium present in “hard” water. But it’s actually quite unusual for enough soluble magnesium to be present at the right amount in the untreated water stream, so it must be added.
Thirdly, the concentration of acid must be low for the chemical reaction making struvite to take place. The biological treatment activity naturally creates enough acid, so it must be removed or neutralized so a buildup of acid does not interfere with the health of the bacteria. Alkaline (high pH) materials neutralize acidic water (low pH) by dissolving and turning the acid into water (neutral pH) and salt. If salt builds up in the system, it can also interfere with the health of treatment bacteria. Or if the treated water is re-used in agriculture, salt can hurt crops and destroy land value.
Fortunately, there’s a way to solve the need to add magnesium, the need to neutralize acidic water, and the requirement to avoid salt buildup. Magnesium hydroxide is an intrinsically safe material that is part magnesium, neutralizes acid, and does not contribute to mineral salt buildup. Other forms of soluble magnesium mineral salts provide magnesium but do not provide alkalinity control.
For the nutrient reuse-recycle process to be beneficial, the fertilizer formed when magnesium, ammonium, phosphate and water combine to form struvite (magnesium ammonium phosphate hexa-hydrate) must stay where you want it, keep away from where you don’t want it, and be processed in a saleable form in a reasonable period of time. An old saw in chemistry is that any chemical reaction is driven forward by time, temperature, and concentration. Take away any of these and the reaction will not proceed. When all three dissolved reactants – magnesium, ammonium, and phosphate – are at the right conditions and concentrations, struvite can be formed and potentially recovered.
There are times and places that we want to avoid or delay struvite formation. For instance, we might want to form free crystals of struvite for removal and agricultural use, but we want to avoid struvite buildup on surfaces (called scale) within the same process. Or, we want to avoid struvite formation in systems where magnesium, ammonia, and phosphate are present, but are not yet equipped to recover and recycle struvite. The presence of dissolved magnesium ions usually is the limiting factor in the struvite reaction. That means selecting the source of magnesium reactant is important for controlling the speed of reaction and whether the reaction will proceed at all.
Magnesium hydroxide provides required alkalinity (acid neutralization) for the biological treatment, but solubility is limited by pH. Magnesium hydroxide is supplied in a slurry form; by definition slurry contains undissolved particles suspended in a liquid (water) medium. The suspended magnesium hydroxide particles dissolve best in acid conditions, dissolve slowly in near-neutral pH, and the particles have very low solubility in the high pH levels needed for struvite creation. That means magnesium hydroxide is an excellent source of alkalinity to support the biological treatment process, but may not provide sufficient soluble magnesium ions to support struvite formation.
To recycle effluent phosphorus and nitrogen into useful struvite fertilizer, alternate sources of magnesium include magnesium sulfate which is highly soluble even at the higher pH levels needed for optimal reaction rates. Put another way, it’s very possible to use magnesium hydroxide for pH and alkalinity control while avoiding uncontrolled formation of struvite scale, which can cause operational problems and decrease efficiency.
Struvite is not the only precipitate that may be formed in wastewater treatment systems. Carbonates, phosphate salts, gypsum, and various other metal precipitates may also be beneficially induced or encountered as scale (Morse, Brett, Guy and Lester). If an unwanted precipitate is present, first determine its true identity before taking “corrective” action.
Struvite formation can be predicted.
Whether struvite is desired or not, the most important factor in the formation of struvite is super-saturation. That is, the dissolved concentration of the reactants phosphorus (as phosphate ion) and nitrogen (as ammonium ion) and cationic magnesium must be high enough and in the proper ratio to encourage the formation of struvite crystals. A 30% excess of one of the reactants has been found to drive the reaction (Carisson, Aspergren, Lee and Hilmer).
|Chemical Symbol||Mg||N||P||Essential elemental reactant|
|Elemental Molar Ratio||1||:||1||1||Present in struvite formula MgNH4PO4*6H2O|
|Elemental Weight Ratio||24||:||14||31||Present in struvite|
|Dissolved Ion Name||Magnesium||Ammonium||Phosphate|
|Dissolved Ion Symbol||Mg+2||NH4+||PO4-3||The form of reactants in solution|
|Ionic Molar Ratio||1||:||1||1||Present in struvite formula MgNH4PO4*6H2O|
|Ionic Weight Ratio||24||:||18||95||Present in struvite|
Struvite is a white inorganic crystalline material that tends to precipitate in places with increased turbulence such as pumps, pipe bends, aerators, valves, jets and nozzles. Physical inspection of the treatment plant and its unit operations can often identify specific locations within the plant where struvite can form, either intentionally or spontaneously. Struvite tends to form where pH values are above 8.5, or where pH increases when dissolved acidic gasses (such as carbon dioxide or hydrogen sulfide) are removed (Doyle, Oldring, Churchley and Parsons). For this reason, covered containment to prevent off-gassing, and pH control can both help minimize struvite formation at critical locations.
Struvite crystals build up on rough surfaces – or where dissolved or suspended particulate impurities are either present or forming – these provide nucleation points where the struvite crystals can begin forming. If conditions are positive for struvite formation, the rate of crystal formation increases as pH increases and as temperature increases. As total dissolved solids (TDS) increase, precipitation initiates more easily and at a faster rate. Struvite is no exception (Ohlinger, Young and Schroeder). Struvite crystal formation around foreign ions has been reported (Ariyanto, Ang and Sen).
To determine the most likely location for struvite scale to spontaneously form In your own facility, or to initiate intentional struvite precipitation, researchers have described an inspection / sampling / testing / analysis approach to determine either the risk of struvite scaling or the best location to initiate a struvite recovery process. For example, researchers found that centrifuge liquors are most favorable to struvite formation (Jaffer, Clark, Pearce and Parsons). Once identified, the appropriate actions can be taken by facilities operators to encourage or prevent struvite precipitation.
Struvite formation can be controlled
Whether the process is anaerobic, aerobic, activated sludge, chemical precipitation, or a combination of treatment methods, the degree to which the process is likely or unlikely to form struvite is testable and can be adjusted or controlled to suit the permit goals. This knowledge can assist the skilled operator to efficiently reduce the level of nutrients discharged to the environment or circulated with treated repurposed water. Know your process, know your limits, know your potential and Reduce – Reuse – Recycle.