Solids are an inevitable challenge in wastewater treatment. In this article, we will share with you 5 Ways to Reduce Wastewater Solids.
Even the world’s most efficient treatment systems must waste and dispose of some byproducts. The types of solids covered in this article are: trash, organic, and inorganic. Materials that enter the collection system in large, complex, masses, such as rags, plastic bottles, and baby wipes, are considered trash.
Organic solids are the sewage and plant matter loads in wastewater. Lastly, inorganic debris consist of sediments, eggshells, and coffee grounds. Furthermore, trash and inorganic material contribute to the proportion of solids that are not biodegradable in wastewater facilities. However, organic solids can be broken down by microorganisms in the treatment process.
Reducing solids in wastewater treatment is essential for meeting effluent quality limits and maintaining proper process efficiency. Having high concentrations of organic solids in treated wastewater discharge can be problematic for meeting federal water quality standards.
The US Environmental Protection Agency can fine wastewater utilities (or Publicly Owned Treatment Works (POTW)) for being out of compliance with National Pollution Discharge Elimination System (NPDES) limits. POTW can be penalized for sanitary sewer overflows (SSO) and combined sewer overflows (CSO) which result from solids blockages in collection systems.
Additionally, solids buildups can seriously reduce the space available for treatment in wastewater facilities. As this happens, the wastewater residence time is decreased, and the process efficiency can decline.
Lastly, trash and inorganic solids can obstruct or add wear and tear to mechanical components of wastewater treatment systems. Issues with these solids can increase plant maintenance demands and add to operation expenses.
The following list discusses five ways to reduce solids in wastewater treatment. Some items on the list are specific to a certain types of treatment process which is identified in the item description (Ex. lagoon system or mechanical plant).
1. Solids Prevention in Collection Systems
The most logical way to reduce wastewater solids is to prevent them from entering the collection system. This is easier said than done, but it doesn’t hurt to try. Wastewater utilities can [a] install solids screening structures in sewer systems or over surface openings and [b] ask their citizens to be mindful of what they throw out the window and put down the drain. These prevention strategies can be used in any community no matter the collection or treatment process design.
[a] Screens and grates can be an effective way to prevent trash and large solids from entering the collection system. Coarse screens (or bar screens) and trash racks (or trash grates) can be placed over sewer drains or within sewer systems. Combined Sewer systems are especially suspectable to having trash blockages.
Because combined sewer collection systems receive stormwater, trash from streets and water ways can more easily enter the collection system. Large floatable trash that gets caught in these systems contribute to CSO. CSO are inconvenient and financially taxing for wastewater utilities.
[b] Community awareness of proper solids disposal and impacts on wastewater management is greatly important for preventing solids blockages and treatment issues. Domestic wastewater is full of solids. Some are easily biodegradable and can be treated while others are plastic or complex paper products that are impossible to reduce with standard treatment.
Items such as baby wipes and plastics caps (the list goes on) should never be flushed down toilets or rinsed down sink drains. For that matter, limiting any form of solid (organic or not in nature) from entering the collection system helps POTW do their job properly.
Through education, the public can learn what they can and cannot put into the collection systems and how to prevent certain solids from going down the drain. Placing a screens over sink drains can provide an easy way to collect solids for disposal. Additionally, having a convenient trash bin by the toilet can remind individuals to throw away baby wipes and floss instead flushing them.
Community outreach can raise awareness of issues with solids at local events. Lastly, providing accessible web and print content on this topic can help the information spread from one mindful citizen to another. Thoughts like, “I can make a difference” and “I can reduce the environmental impacts of my wastewater” can empower individuals and eventually whole communities.
Benefits:
[a] Screening methods work well for preventing trash in collection systems. They are also affordable and can be added into most municipal wastewater budgets.
[b] Education on solids in wastewater management can change common trash disposal practices and empower communities to reduce their environmental impacts. This prevention method is affordable and easy to implement using the internet and social media.
Limitations:
[a] Trash screens and grates require daily cleaning to allow flows to move through them. Solids that are collected from screening devices need to be transported to a landfill. This process requires additional cost and handling.
[b] Whether community members will follow suggested solids disposal practices (or not) is completely up to them. Reducing solids with this method can take time and persistence from POTW.
…Continuing with 5 Ways to Reduce Wastewater Solids
2. Establishing a Headworks
Establishing a headworks comes in at number 2 on our list in our “5 Ways to Reduce Wastewater Solids” article.
Wastewater headworks systems are comprised of screening and grit removing equipment. Headworks are installed to remove [a] suspended (or floatable) solids/trash and [b] inorganic solids prior to the treatment process. This form of pretreatment can be implemented ahead of any type of wastewater treatment system no matter the size or design. However, these specifications do impact the effectiveness of the headworks equipment installed.
[a] There are several types of screen devices that prevent large solids and trash (including plastics, paper products, and woody debris) from moving passed the headworks to the treatment system. These include (but are not limited to): manual bar screens, automatic bar screens, and all-in-one headworks machines. Standard manual bar screens use vertical bars with 1 to 2 inch spacing to catch trash and debris.
The solids caught in these types of screens must be raked manually to remove. Second, automatic bar screens are similar to manual bar screens but are equipped with mechanical raking capabilities. Third, all-in-one headworks machines (like the Auger Monster system from JWC Environmental) combine screening, grinding, and compacting technologies to remove solids.
Equipment of this type mechanically grinds solids in wastewater streams, pushes the grinded solids through a fine screen (2 to 6 mm), and compacts the leftover solids from the screen. Material in the compacter can then be easily disposed.
[b] Grit chambers remove inorganic solids in headworks systems. Reducing inorganic materials during pretreatment is important, because these solids contribute to mechanical issues and increased solids disposal costs for wastewater utilities. Standard grit chambers have long, circular tanks that are designed to slow the velocity of wastewater.
When the velocity is decreased, sediments and other inorganics settle out and are collected for disposal. Other designs use vortex technology which circulates wastewater so that inorganic solids quickly settle to the bottom. Grit cambers are cleaned both manually and automatically depending on the equipment type.
Benefits:
[a] Biodredging is highly effective in wastewater lagoon systems because of the long residence time of wastewater. This process reduces the cost to mechanically dredge these facilities.
[b] Bioaugmentation reduces organic loads in mechanical plants. Solids settling can also be improved for easier handling and recycling.
Limitations:
[a] Consistent doses of biology need to be added to the lagoon system to achieve the best results. This increases operation costs.
[b] Biological solids reduction in mechanical plants is most limited by wastewater residence times. Wastewater is processed for shorter amounts of time in these systems, so solids reduction can be less or can take longer.
4. Mechanical Dredging
As we move to the 4th item in our “5 Ways to Reduce Wastewater Solids” article, we now come to Mechanical Dredging.
This method of solids removal is specifically used in wastewater lagoon systems. Lagoon facilities are not designed to reduce solids (or sludge) within the treatment process. Therefore, the solids in lagoons settle to the bottom and accumulate leaving treated wastewater at the surface free of particulate.
The excess solids can be reduced biologically (as discussed above) or mechanically. Mechanical sludge removal is call dredging. This process involve [a] removing the sludge and [b] disposing of it.
[a] A construction digger is used to dredge the wastewater solids at the bottom of lagoons. This machine digs out sludge to increase the wastewater holding capacity of the process. Treatment efficiency is increased after dredging because the wastewater microbes in the system have less to digest. Mechanical dredging typically occurs annually, but the frequency of solids removal depends on how quickly solids accumulate.
[b] Once sludge is dredged from wastewater lagoons, the material needs to be dewatered and hauled to a landfill. Solids are squeezed through a dewatering machine to remove all liquid contents. Next, the dewatered sludge is loaded into trucks and hauled to a landfill.
Benefits:
[a & b] Mechanical solids removal is highly effective and relatively fast. Machines can dig out years of sludge buildup in a matter of days. Additionally, this form of solids removal is performed about once a year.
Limitations:
[a & b] Mechanical dredging is expensive and can exceed small community wastewater budgets. The cost of hauling dewatered sludge is $300 to $500 per dry ton.
5. Solids Wasting
As we round out our “5 Ways to Reduce Wastewater Solids” article, we have come to the last item. Solids in mechanical activated sludge plants are handled within the system design.
At the end of the wastewater treatment process, solids [a] settle out of solution and [b] are wasted from the return activated sludge (RAS). Wasted solids are removed from the system and dewatered. After dewatering, solids are disposed in a landfill or repurposed. The RAS is then fed back into the system to supply the biology needed for treatment. The quantity of solids wasted vs returned in the plant is dependent on the organic loadings received and the process efficiency.
[a] Mechanical activated sludge plants use secondary clarifiers to settle out solids (including alive and dead biomass). These tank structures gently reduce wastewater velocity so that suspended matter can settle to the bottom. Biosolids are collected at the bottom of the tank and are wasted or become RAS.
[b] Wasted solids are sent to a dewatering system. Removing the liquid from the activated sludge reduces the cost to haul and dispose of the waste. These solids are transported to a landfill or used as compost or food in anaerobic digestion.
Benefits
[a & b] Activated sludge processes are effective in reducing suspended solids among other wastewater constituents. Solids wasting can be used to balance these processes and increase treatment efficiency. Lastly, wasted solids can be repurposed to feed anaerobic digestion. The biogas produced during anaerobic digestion can be used as a fuel source for plant operations.
Limitations
[a & b] The activated sludge process is dependent on a settling biomass. FB often grow abundant in these systems preventing proper settling from occurring. In such circumstances, solids are not properly wasted or returned to keep the process going. Additionally, hauling solids for disposal is expensive.
Closing Thoughts
The items on the list that can help reduce solids in any type of treatment facility include prevention through education and screening, installing a headworks, and solids bioaugmentation. Mechanical dredging and solids wasting practices are specific to lagoon systems and activated sludge processes.
Moreover, augmenting wastewater treatment systems with cultures of bacteria can help improve settleability in mechanical plants. Bioaugmentation can also reduce wasted solids and sludge hauling costs for wastewater facilities (lagoon and mechanical). There is no one way to manage solids in wastewater treatment. The above list can be used as a tool to understand several of the options available for reducing solids.