Septage Receiving System Design Best Practices
Best Practices for Designing a Septage Receiving System
Many wastewater treatment plants and other facilities make the decision to process septage before it enters the treatment process. They do so with a septage receiving station, which removes trash, with a compaction stage, which dewaters and reduces the volume of solid material produced. However, if the septage receiving system is not designed properly, it cannot deliver on the benefits. This blog post discusses some best practices for system design.
Benefits of Screening
Screening wastewater influent is a critical first step to ensuring longer equipment life and offers many benefits, including:
- Prevention of fouling downstream – Large debris, grit, and rags can create havoc on downstream equipment by causing reduced flow and performance. Rags can accumulate around weirs and gates causing problems with flow distribution.
- Prevention of fouled pump internals – Screening removes the causes of undue wear on pump internal components, such as impellers or mixing blades.
- Less/no debris floating in clarifiers – Screening expedites the clarification process by removing solids from the water beforehand.
- Minimizing fouled aeration.
- Less/no inorganics in wastewater sludge – Removing solids from the influent removes much of the inorganic solids, making it possible for the remaining organic material to be broken down through microorganisms in the treatment process.
Septage Receiving System Configuration
There are several ways to configure septage receiving equipment. Typically, a system uses a tank mounted screen and compactor. Stainless steel is commonly used for construction to withstand a corrosive environment.
The best septage design utilizes the optimum grid type, screen type, and compaction approach to process the the influent waste stream. Septage is arguably the toughest waste stream input to the wastewater process due to the variable nature of content including rags, rocks, heavy grit, and higher solids content. The offload of material captured on the screen and removal of rocks are also important for successful operation.
Large Debris Removal: Given the potential issues septage represents at the wastewater facility, it is critical that large debris, such as rocks, plastic cartons, plastic wrapping, wipes, rags, clothing and household waste are removed before they have a chance to clog or damage pumps, valves, and other downstream equipment. These items are removed by coarse screens, typically either manually or mechanically cleaned bar screens, with openings of 6 mm (~0.25 in) or larger.
There are many screening grid types available for typical wastewater screening, including slotted link, circular or flat bar, stainless perforated plate, or polymer perforated plate.
Screen Grids (Bar Grid / Perforated Plate Grid)
- Manual bar screens (or “racks”) are found primarily in older wastewater treatment plants. The bar spacing is about 30-50 mm (~1-2 in). Although very inexpensive, these screens are inefficient and require constant attention by staff to manually clean the rags and debris frequently.
- Mechanical bar screens have spacing of about 6-38 mm (~0.25-1.5 in) and are found in newer installations. They clean themselves mechanically, making them more expensive but much easier to maintain.
- Reciprocating rake (or climber) screens have a rake that starts at the base of the screen and pulls off the debris as it makes its way to the top, where the screenings are removed. These screens are expensive but are self-cleaning so they require less manual labor to keep clean.
- Cylindrical fine screens offer better performance to capture solids. Cylindrical fine screens are constructed as either perforated plates formed into a drum design or cylindrical bar screen slotted designs. Slotted designs are preferable over perforated plates when used for wastewater septage screening as the screen blinding factor is less with high solids content.
- The through flow band screen design is similar to the circular bar design when used for septage screening. The through flow band design utilizes a stainless steel bar grid. During normal operation, the grid sits stationary in the influent flow channel allowing solids to mat on the grid surface. Once the upstream water level reaches a set point the screen grid travels to remove the dirty grid and carry solids to compaction. A spray wash is used to remove and wash solids from the screen grid. Septage waste with high levels of rags and trash present can blind perforated holes at peak flows, while link and bar-style grids have more vertical spacing that can perform better with high flow and high blinding conditions.
Rock Removal: Rock removal is also an important feature of septage receiving systems. Some screen types cannot unload large rocks that enter the system, so a rock trap must be used with these types of systems. Rock traps are generally steel vessels with a capture area designed to trap large objects by settling into the bottom of the vessel. Rock traps generally require daily maintenance to remove accumulated material. Close contact with sewage is generally not acceptable if it can be avoided in most treatment operations.
Compaction: Compaction occurs after large solids are removed. Following is the process and equipment used:
- Once large solids are removed by the screen, they are dewatered and compacted before disposal.
- Sludge is reduced in size and water content through compacting. Several types of compactors are typically used in septage service including shafted auger screw compactors, shaftless auger conveyer/compactor designs and shafted auger conveyer/compactor designs.
- Shaftless conveyer/compactor designs are generally less expensive than shafted designs but are also do not have higher torque available to clear the screening grid if an increased load occurs in the process.
- Some compactors use just one motor for screening and compaction. These types of designs have less operational flexibility to adjust screen processing if a heavy load is experienced or are limited in torque to dewater solids. Designs that utilize one motor for screening and compaction are typically less expensive.
- Compactors can be designed with a washing section to wash organics out of the solids back into the bulk wastewater flow. Removal of organics is important for odor control and some small plants require the nutrient load supplied by organic content.
In this article we have reviewed the best practices for designing a septage receiving system, for further information about the challenges of septage receiving stations read our next blog post on the topic.
If you would like to discuss your specific needs, connect with Hydro-Dyne’s highly experienced professionals for more information about our Dusky Shark Septage Receiving Station.