Phosphorus Harvesting

Phosphorus recovery goes beyond resource recovery and reuse of nutrients, it touches the very core of life; Of all known life, none is possible without the nutrient element phosphorus. Phosphorus is an essential component of cellular walls and membranes, as well as for DNA, RNA, and for cellular energy transfers via ATP. Recovering and reusing this critical element not only helps alleviate excess phosphorus on land (applied as part of the biosolids program) and in waterways, but also stretches the world’s dwindling high quality phosphorus reserves. For this recovery effort, the District selected Ostara Nutrient Recovery Technologies to team with, and the recovery facility at Nine Springs is the sixth full-scale operating facility for Ostara. There was extensive press coverage from the 11th Addition Launch (grand opening of the phosphorus harvesting building).

/Portals/0/UltraMediaGallery/761/6/thumbs/3.photo1.jpg
The District employs biological phosphorus removal to remove phosphorus from water and concentrate it into the micro-organisms. These microorganisms are then removed from the system as waste sludge, taking most of the phosphorus with them.
10/3/2013 2:32:00 PM
The waste sludge is sent to a tank where it is mixed gently with a small amount of acid-phase sludge (the arched cylindrical digester in the background is an acid-phase digester) to promote the release of the phosphorus taken up by the microorganisms during biological phosphorus removal.
10/3/2013 2:32:00 PM
The combined waste sludge and dissolved phosphorus are then directed to a gravity belt thickener where the solids are thickened prior to anaerobic digestion, and the phosphorus rich liquid filtrate is captured.
10/3/2013 2:32:00 PM
This is the underside of the gravity belt thickener (the blue belt is visible at the top), showing the fairly clear phosphorus-rich liquid that was separated from the solids. This is what we will harvest phosphorus from!
10/3/2013 2:32:00 PM
These are two of the reactor feed pumps that move the filtrate from the gravity belt thickener building to feed the phosphorus harvesting reactors.
10/3/2013 2:32:00 PM
The phosphorus sent to the reactors is in a dissolved form, and we need to form a solid particle precipitate to separate it from the liquid. A reactor is used to create the ideal conditions to form this solid particle precipitate which is called struvite, which consists of equal chemical molar ratios of magnesium, ammonia, and phosphate. Our two reactors are called the Ostara Pearl 2000 series. We have two reactors that are projected to handle our phosphorus harvesting needs through 2030.
10/3/2013 2:32:00 PM
Flow enters the reactors from the bottom (along with a recycle stream) that lifts the bed of struvite particles, called prills. As the dissolved phosphorus moves upward through the reactor, chemicals are added to optimize the conditions for the formation of struvite. This struvite forms on the outside of the prills until they reach the desired size and some are harvested off. The specific size selected for harvest is determined by demand within the fertilizer industry.
10/3/2013 2:32:00 PM
After harvesting the prills, they pass through a screen to separate them from most of the water. Pictured here is the product dryer, which uses heated air to complete the drying and prepare the product for storage. We actually use hot water (heated by biogas) to provide the heat required and save energy.
10/3/2013 2:32:00 PM
This is the bagging structure; four large bags hold struvite product of varying size classifications until we have enough to fill “smaller bags”. Our smaller bags are 1-ton super sacks, which are inconvenient to carry home, but they do work well to load onto a flatbed for transport in bulk to Ostara.
10/3/2013 2:32:00 PM
This is what the end product looks like, sort of pearl colored and generally a few millimeters in diameter. Of course, the specific size can be adjusted by adjusting the set points of the reactor. Did you know about 10% - 15% of kidney stones are made of struvite?
10/3/2013 2:32:00 PM
Uncontrolled struvite formation occurs in many treatment facilities, including ours, and can be quite a problem. Pipes can become choked off and valves difficult to impossible to operate. One added benefit we hope to gain from this process is to reduce the nuisance formation of this compound, and thus save time and money in maintenance!
10/3/2013 2:32:00 PM
Some chemical addition is required to create ideal conditions for struvite formation. Here is where we deliver the magnesium chloride and sodium hydroxide. Magnesium chloride is used to provide enough magnesium to maximize product production. Sodium hydroxide is used to raise the pH to the ideal precipitation point.
10/3/2013 2:32:00 PM
This is a storage tank for the sodium hydroxide. Magnesium chloride is relatively common and less reactive, whereas sodium hydroxide is fairly reactive and caustic. Because of this, the sodium hydroxide storage is in a separate room.
10/3/2013 2:32:00 PM