Introduction
Large Dairy producers have a substantial amount of manure slurry to deal with from the production of milk and other dairy products. Effective manure management plays a critical role in optimizing nutrient recovery and minimizing environmental impact during the filtration process. In the past, the manure slurry was either hauled off and land applied as fertilizer or stored in local ponds. However, there are governmental restrictions as to when and where this slurry can be land applied, making it increasingly difficult for the dairy producers to deal with.
Solids from drum filter.
More recently, after initial separation using traditional drum filters, the remaining manure slurry gets hauled off and taken to anaerobic digesters to create “green” methane gas. This has been increasingly desirable due to the introduction of green energy credits in the USA. What was previously a waste stream for dairies has become a boon for the farmers after anaerobically digesting the manure slurries to create methane gas. Manure management in this context has proven beneficial.
Waste Management
The cost of hauling manure slurries for offsite disposal or land application can be expensive, and sometimes environmentally irresponsible. In the states that still allow land application of manure slurries, this usually cannot be done when the ground is frozen during the winter months. Due to the volume of manure slurries generated by the larger producers, hauling can be increasingly cost prohibitive.
The introduction of anaerobic digestion of the manure slurries has already solved many problems for the dairy farmers, although the hauling of these slurries to the digesters, which may be far away, can still be expensive. Effective manure management in this system reduces costs significantly.
Benefits of VSEP
Manure slurry after drum filter.
New Logic Research was contacted to test the potential benefits of introducing a VSEP system to filter the manure prior to anaerobic digestion with the goals of reducing the volume of manure slurry that has to be hauled to the digester, increasing the percentage of solids to increase the efficiency of the digesters, and providing a clean water stream to be used as wash-down water and drinking water for the cattle in the dairy. This is a crucial part of manure management for the operation.
Long-term pilot testing was conducted at the site using an 18” VSEP system. Many batches were concentrated through the VSEP system. The permeate from the VSEP system was polished through a traditional spiral-wound reverse osmosis (RO) system. After the pilot test report was issued, the customer purchased a full-scale 6 unit 84” VSEP system with an RO spiral.
VSEP Manure Slurry Process
After the existing drum filter, the remaining manure passes through a two-stage SWECO vibrating woven screen filter, first through a 150-micron screen followed by a 250-micron screen to remove any remaining large solids and sand that made it through the drum filters. This slurry is then sent to one of two batch tanks where it is heated up to 30-40 degrees Celsius. The contents of one of the batch tanks is sent through the VSEP system. During the initial batch process, the permeate is sent directly to the Spiral RO tank while the concentrate is returned to the batch tank until the overall recovery set point is reached. Once the desired batch recovery set point is reached, the concentrate is then sent directly to the final concentrate tank until the batch tank is empty.
Installed 6 unit VSEP system.
Meanwhile, once the Spiral RO feed tank reaches a high-level set-point, the Spiral turns on automatically and sends its permeate to the final permeate tank at a fixed recovery set-point to be used as VSEP/Spiral RO cleaning water, in the plant as wash-down water, boiler feed water, or drinking water for the herd. The permeate could also be used for dust control on roads. The spiral concentrate is returned to the VSEP batch tanks.
Results
Batches run to 55% recovery and can process 70,000 gallons of feed per day.
VSEP Stage:
Starting feed solids: 3.5% (TS)
Ending concentrate solids: 6.8% (TS)
Spiral Stage:
Starting feed conductivity: 1,200µS-1,800µS
Ending permeate conductivity 200µS-400µS.
Installed Spiral Stage 2.
Summary
The benefits of introducing the VSEP system to this large dairy producer are as follows:
· Reducing hauling costs of manure slurry to digesters.
· Increasing the efficiency of methane production by increasing solids to be digested.
· Creating a new high-quality permeate stream that can be used for various purposes.
VSEP and Spiral RO system.
An unforeseen benefit of the VSEP system at this plant is that the VSEP concentrate is very desirable for the digester start-up. They are referring to the VSEP concentrate as “activated”. When introducing the VSEP concentrate to the digesters, it was reported that methane production starts much more rapidly and yields more methane than without the use of VSEP Concentrate. Effective manure management is key in maximizing these benefits.