Sewage sludge is the product left over after sewage has been treated at a wastewater treatment plant. At the treatment plant, wastewater undergoes a complex process involving a variety of steps used to treat and clarify the wastewater so it can safely re-enter the environment. While this system does a remarkable job of cleaning and recycling the water back to the environment, the resulting sludge, also commonly referred to as biosolids, still needs to be processed.
There are a number of disposal and treatment methods employed at sewerage districts around the country. And while technology has come a long way in developing sustainable disposal methods, there is still opportunity for improved methods that maximize the opportunity to utilize biosolids as a value-added product. Below are some of the current methods commonly used to dispose of biosolids, as well as an explanation on the opportunity for improved applications.
While a modern landfill is a safe and reliable approach to disposing of waste solids, there are a number of downsides with this method.
Perhaps the most critical issue is that landfilling waste solids disregards the valuable nutrients and organic matter that the collected sludge holds, ultimately wasting a potentially valuable material. Many of the nutrients that are thrown away in the landfill with the biosolids are non-renewable, requiring fossil fuel inputs to produce more. The ability to put these nutrients back into the ecosystem would not only be a more sustainable alternative to producing more, but would also avoid the material taking up space in a landfill.
In addition, hauling wet solids to the landfill can be a costly endeavor. Utilizing the waste material as a value-added product would provide a revenue stream where waste disposal costs were previously incurred.
Wastewater treatment sludge can also be disposed of through combustion in an incinerator. Incineration is a particularly viable approach where available land for application is limited.
This capital-intensive approach reduces the biosolids to a small amount of ash, which is then typically sent to a landfill. While the ash may have a very high nutrient value, the metals also tend to concentrate in the ash, limiting its marketability. While FEECO has supplied ash granulation systems to add value to this waste stream and produce a granular ash product, air quality and permitting issues associated with incineration have made this approach less common, and this trend is expected to continue.
Composting is a treatment approach that offers low capital and operating costs for processing. The land requirement to properly windrow is significant, however, as the required retention time is typically around 6 weeks.
The major disadvantage to this approach is that the product is typically of little resale value, usually around $10 – $20 per ton. Despite being a wonderful soil amendment, rich in organic matter and micronutrients, the compost is in a physical form that cannot be readily incorporated into existing product lines and is difficult to transport, ultimately limiting its marketability. There is a solution to this problem, however: organic waste granulation, or organics granulation. Organics granulation transforms a material that is difficult to handle into an easy to use, dry granular product that can be easily incorporated into existing product lines.
The key pieces of equipment required for organics granulation are a FEECO Paddle Mixer and Rotary Dryer, shown below. Once granulated and sized to meet fertilizer standards, the compost can be handled and spread with standard equipment. The compost can also be bulk blended with chemical fertilizers, greatly increasing its marketability and value.
Drying and Granulation
For most sewerage districts, the ideal solution for wastewater sludge processing is the production of high-value fertilizer granules. Similar to the granular compost approach, a granulation facility produces a high quality fertilizer granule from the sludge. A simplified process flow diagram is shown below.
The image below shows a simplified organics granulation system with a mixer-dryer setup. Click image to view larger.
|Flow Diagram Key|
|A – Raw Feed
B – Binder
C – Paddle Mixer
D – Fuel
E – Burner
F – Rotary Drum Dryer
G – Rotary Cooler
H – Baghouse
I – Stack (to Atmosphere)
|J – Product Screen
K – Product Diverter
L – Crushing Equipment
M – Recycle Bin
N – Baghouse
O – Stack (to Atmosphere)
P – Product Bin
Q – Load Out, Packaging Line, Stock Pile
The granulation process begins in the paddle mixer, which back-mixes dry, off-specification material with the incoming wet sludge. The wet granules produced in the paddle mixer are then dried in a rotary drum dryer. The dryer provides secondary granulation through the tumbling action at the feed end of the dryer, and also provides efficient drying of the granules. The dry pellets are then screened to produce a fertilizer-grade product, with the off-spec material recycled back to the granulation circuit.
The major operating expense of the granulation plant is the heat source required to dry the fertilizer product. However, a significant advantage of the granulation approach is its flexibility in the utilization of available waste heat; the system can effectively use waste heat from an existing incinerator, biogas generated from anaerobic digesters, or other sources of waste heat.
The fertilizer granules produced in a FEECO granulation plant typically have pellet hardness values in the range of 4-6 lb/granule, and a size of 2-4 mm diameter. This is sufficient for most handling and blending operations. A binder can be added to increase the strength, as needed for special cases.
The modern wastewater treatment plant is an invaluable public resource. However, opportunity exists to utilize the valuable nutrients and organic matter contained in sewage sludge, with the most promising opportunity being organics granulation.
FEECO has been providing the best in granular fertilizer systems for both inorganic and organic applications for decades. To learn more about organics granulation for biosolids, contact us today!