Struvite precipitation in wastewater

Chemical formula struvite

In almost all organic wastewater one will find struvite. Organic wastewater is mainly human urine, dairy manure or other livestock wastewater. The chemical formula for struvite is (NH₄)MgPO₄·6H₂O. Struvite belongs to the mineral class of phosphates. The full chemical name of Stuvite is ammonium magnesium phospate. It is one of the particularly poorly soluble compounds of ammonium and magnesium. If heated or dryed, the struvite formation turns white, becasue it loses water. This white struvite precipitation is the nightmare of the operators of the wastwater treatment plant. This struvite formation becomes very hard, becasue to the struvite crystallization. For a lot of customers it is often easier to replace the entire pipes than the removal of the struvite and cleaning of the pipes.

The actual case is the wastewater treatment plant of the city of Kranj. Kranj has 40.000 citizens and is the third biggest city of Slovenia. Among skiers Kranj is famous for the World Cup in Kranjska gora.

Struvite precipitation

The plant is newly built and uses the latest technology. All pipes and the tubular heat exchanger are made of stainless steel. In the final step of the wastewater treatment process, the sludge of the wastewater contains only very little water. This sludge is heated to 37° in order to extract methane out of the sludge. The methane is used to generate electricity. There is a lot of struvite precipitation in the heat exchanger. Because of the higher temperature, compared to the rest of the wastewater treatment plant.

Struvite causes problems in heat exchangers

The precipitated struvite has two negative impacts.
First,
the temperature in the heat exchanger is difficiult to reach and to hold, as more struvite percipitates. This leads to higher energy consumption and to not optimized extraction of methane.
Second,
the inner diameter of the pipes is gets lesser over time. This reduce the flow and the amount of wastewater, which can pass the through. This requires cleaning and removal of the struvite or to replace the equipment.

How the performance was checked

The customer used to check the perfomance of the installed 5 inch Merus Ring the temperature of the wastewater. The pipes have been several times X-rayed, to monitor the development of the struvite precipitation. The heat exchangers have been checked visual before the installation and 3 month after the installation.

the customer wrote:

the difference between before and after using Merus Ring is obvious. After installing the Merus Ring, we dont need to replace the pipes anymore. Only effortless periodical cleaing of the pipes and heat exchanger is needed. Consequently we save on maintenance cost and have a smooth process.