By Crista Renouard
From the earliest days, water has been a convenient and efficient way of conveying collected wastes to a centralized location so they could be processed. Since then, great strides have been made transitioning the role of wastewater treatment plants (WWTPs) from simply capturing and treating collected sewage into becoming a valuable resource generator. As populations increased and climate conditions stressed the availability of water, it became practical to develop techniques to harvest the transport water for reuse back into the community.
When looking at the operational costs of a wastewater treatment plant, the top two categories of expenditures are energy and disposal costs. By taking a closer look at existing techniques being employed in a WWTP it is possible to continue lowering operational costs. In many cases, it is possible to actually pay for newer technologies with the savings realized through that process of optimization.
The makeup of material for disposal in a WWTP is typically comprised of screenings captured at the headworks, grit collected in the settling chambers, and sludge separated in the clarifiers and digesters. The trend in WWTP design is to move to finer levels of both screenings and grit capture. This is largely driven by the need to protect equipment and technology being employed further into the process to achieve increasing treatment objectives and performance goals.
What is entering the headworks?
The screen type selected for the primary screening in the headworks can have a radical effect on the actual quantity of screenings captured and isolated out of the waste stream flow. For instance, going from a 12 mm bar screen to a 6 mm bar screen can double the volume of screenings captured.
As the screening elements get smaller and the screenings capture efficiency increases, higher levels of organics are captured along with the inert screenings debris. Not only does this affect the volume of the screenings, it also removes valuable energy-laden organics that could be used in digestion and nutrient removal processes.
By looking at the captured screenings and how they are processed for disposal it is possible to greatly reduce the volume to be disposed. Additionally, the organics can be separated and returned to process. Wash presses play an integral role in processing the captured screenings prior to disposal.
Weight reduction is key
The financial offset on disposal fees can be significant through effective use of wash presses. For instance, as a theoretical example, an average tipping fee for municipal waste for landfill is $49 per wet ton. For the purposes of this exercise we will use a weight unit of 40 lbs per ft3 raw wet screenings collected. Assuming 20 ft3/Mgal screenings capture, the screenings produced for a 10 MGD plant with ¼” (6mm) bar spacing should be as follows: 10 MGD x 20 ft3 = 200 ft3/day (4 tons/day). Theoretically the raw untreated screenings would cost $71,540 annually to send to landfill. However, there are very few WWTPs that would send raw screenings to landfill.
A wash press has the ability to reduce volume and weight through washing and compaction. A basic wash press can produce a weight reduction of raw screenings approaching 70%, resulting in an estimated tipping fee of $21,462 per year (an annual savings of $50,078 compared to simply landfilling without treatment). From the perspective of process optimization, by going to an additional level of processing using a laundering component to remove higher levels of organics, in addition to the compaction of a conventional wash press, this machine can possibly increase performance as high as 85% reduction of weight. This extra 15% of weight reduction performance improvement would result in additional savings compared to a basic wash press, resulting in a tipping fee of $10,731 per year.
Grit is another area where optimization can provide operational savings. Quantifying the actual volume of grit entering the plant is much more elusive. Grit is less predictable because its presence has more to do with the location of the facility and the condition of the related collection network. The actual potential of grit concentrations in the incoming stream to a wastewater plant can range from 0.53 – 5.0 ft3/Mgal. Much of the current debate is about the size range of the grit particle a given technology can actually remove. Some technologies currently have the ability to capture down to >75 micron.
In an immediate sense, the issues caused by grit are the effects present if it is not captured. Left free to enter the plant, grit ends up filling space in the bottom of clarifiers, aeration basins and digesters. The abrasive quality of grit also prematurely wears down pumps and valves, adding to operations and maintenance overhead.
The actual makeup of typical grit extracted out of a grit basin can have over 50% water and organics. That volume can be reduced, and the capture of actual grit optimized, with the use of a grit classifier washer. By classifying and washing the grit, water and organics are returned to the process without losing the grit, making it possible to cut the grit disposal volume in half.
In certain cases, this technique allows the cleaned and classified grit to be used as landfill capping material. One installation was able to optimize volume of the grit for disposal as well as the classification of the material, resulting in a 79% tipping fee decrease. The leftover material could be used as a landfill cap.
Small changes add up
The handling and disposal of sludge is second in line regarding major overhead considerations for a wastewater treatment plant. But, sometimes by giving a closer look at related processes, small changes can add up to big savings through process optimization.
This article appears in ES&E Magazine’s August 2019 issue.