Niagara Falls: from honeymoon to Love Canal and back by RR Roll

Wasteland to parkland: the Cherry Farm/River Road Remediation by JG Goeddertz, JH Kyles, MS Raybuck

Niagara River toxics 2000 by Niagara River Secretariate

Involving youth in water quality issues by J Spisiak

Remedial action plans in Lake Ontario Basin

Coarse monomedia filtration: a solution to wet weather flow by BT Smith and KM Miller

Water resources management history project by RD Hennigan

Managing Mercury in Erie County by MC Rossi

People and places

President's message by AJ Zabinski

Executive director's report by P Cerro-Reehil

WEF news


Fall 2000 — Vol. 30, No. 3

 

Coarse monomedia filtration: a solution to wet weather flow

by Bryan T. Smith, P.E. and Karen M. Miller

Quick reference
- Why change?
- How do traditional filters work?
- Pioneering a new technology
- Measuring the sands
- Niagara County's experience
- What's next?

Can changing sands provide a solution to the wet weather flow treatment limitations experienced by many New York State wastewater facilities? In 1998 the Town of Tonawanda Wastewater Treatment Facility, in cooperation with the New York State Energy Research and Development Authority (NYSERDA), studied this question by replacing Tonawanda's sixteen traditional mixed-media sand filters with deep bed monomedia filters at their 32-mgd wastewater facility in western New York. The demonstration project, conducted with the assistance of R&D Engineering, PC of Buffalo, was designed to measure the resulting energy reductions, environmental benefits, effluent quality, cost-savings, and treatment capacity enhancement resulting from the conversion to the new filter media. The media had never before been used in New York State.

Figure 1. Media size comparison

Why change?

Why was John Camilleri, Tonawanda's Water Resources Director, so interested in converting to the new silica sand filter media? Like many wastewater facilities throughout the state, the Town's filters were overburdened during high wet weather flows, sometimes necessitating partial filter bypasses that obviously reduced the overall quality of plant effluent during these peak flow events. Mr. Camilleri wondered if the new media could reduce backwash frequency and extend filter run times, thereby increasing treatment capacity during critical wet weather flows. NYSERDA officials knew that finding the answer to this question could be beneficial not only to Tonawanda but to other wastewater facilities throughout the state and, therefore, offered to fund part of the study of this innovative technology.

How do traditional filters work?

During the 1970s the USEPA, through the Clean Waters Act, mandated new standards for wastewater treatment. The more stringent guidelines for wastewater discharge were based on the public access and use of receiving streams. Many municipalities throughout New York State had to upgrade or construct new wastewater treatment plants. Depending on the water quality standards of receiving streams, many of these new plants required the addition of tertiary treatment or, simply stated, sand filters. Based on Descriptive Data of Sewage Treatment Systems in New York State, a 1992 publication by NYSDEC, 152 or nearly 27% of the State's wastewater plants used tertiary treatment to meet stream discharge standards.

Originally, the design of wastewater sand filters was based on similar filters used in water treatment plants - filters specifically designed to treat potable water. Each filter is made up of layers of various grades of fine media (0.2 to 1.0 mm in diameter), most commonly anthracite and fine-grained silica sand. During normal filter operation, secondary effluent wastewater with suspended solids particles is applied to the top of the filter, and water is drawn by gravity through the filter media. (Gravel layers support the media.) The water settles into the filter tiles or underdrain, leaving behind suspended solids, and is collected in a clearwell or reservoir before discharge.

Because these filters were designed to treat relatively clean water, their application to wastewater treatment has posed problems. In wastewater application, the larger solids tend to be strained out on the surface or within the top few inches of the sand filters, quickly reducing filter capacity and requiring frequent backwashing to remove solids. (Backwashing produces a reversal in flow from the bottom of the filter through the gravel layers to suspend and agitate the filter media layers and flush accumulated solids from the filter.) Filter run time, which is the amount of time that each filter may remain on line filtering suspended solids before backwashing is required, varies greatly based on many factors including media age, media quality, and the suspended solids' concentration of water applied to the top of the filter. For a conventional fine-media filter, typical run times can range from 6 to 36 hr between backwashing but can be decreased significantly during heavy wet weather flows when they are most needed to be online.

The addition of air during the backwash process is referred to as "air scour" and significantly increases the ability for the backwash operation to clean the filter, often reducing the amount of backwash water required. The addition of air can also cause filter upset, however, a disturbance that occurs when air pockets become trapped in the layers of filter support gravel. Support gravel upset can cause media to settle into the gravel layers and the filter underdrain, clogging the filters. Despite underdrain and support gravel design changes over the years, many facilities still experience problems with conventional filters.

Pioneering a new technology

In late 1998, to find a solution to some of the problems associated with conventional filters, the Town of Tonawanda began testing a new technology developed by Tetra Technologies, Inc. of Pittsburgh. This innovative filtration process uses a deeper single layer of larger sand particles (3 mm) in combination with a simpler support gravel configuration that reduces the possibility of filter upset. The new system is gaining acceptance in some areas of the country as an energy-efficient alternative for providing tertiary treatment. Before the Tonawanda project, it had only been used in nitrification/denitrification applications in New York State.

Coarse monomedia filtration features a single deep layer (60 inches or more) of large sand particles (2.3 - 3.0 mm) that drive a different mechanism for solids removal. Figure 1 compares the difference in media sizes between coarse monomedia (right) and the fine mixed media used previously in Tonawanda.

During a filtration cycle with coarse monomedia, solids attach to sand particles in the upper layers where void spaces are empty and media surfaces are exposed. As solids continue to build up, pore spaces become restricted, which increases local velocities and drives solids deeper into the filter bed. The larger sand size and bed porosity avoid surface plugging and provide for vastly increased solids capture capacity, which leads to significantly longer filter run times between backwashes.

Figure 2. Empty cell

The objective of the Tonawanda/NYSERDA demonstration project was to evaluate the technical and economic effect of using the new monomedia process in wastewater plants throughout New York State by applying it to Tonawanda's existing sixteen filters, which process 32 mgd of wastewater. R&D Engineering and the Town hoped to find answers to important technical and economic questions regarding the use of the new process:

  • How would the new monomedia filters' effluent quality in both dry and wet weather conditions compare with conventional mixed-media filters?
  • Would the monomedia filters improve maximum and minimum filter run times?
  • Would the new technology increase plant wet weather flow treatment capacity?
  • Would energy use be reduced significantly, and would cost be reduced as a result of decreased filter backwash frequency and backwash water flow rates?
  • What other benefits would result from reducing total plant backwash water recycle volume

Early results of the pilot project were compelling enough to impress the nearby Niagara County Sewer District. Frank Nerone, District Executive Director and Superintendent of the County's wastewater plant, worked closely with John Camilleri and Tonawanda's Plant Superintendent, Ken Maving, in initially researching the coarse monomedia concept. Mr. Nerone immediately began the process of converting Niagara County's conventional filters to coarse monomedia with water and air-scour backwash in the spring of 1999.

Figure 3. Monomedia installation

Measuring the sands

When installed in 1993, Tonawanda's sixteen mixed media filters could treat nearly 2.5 mgd per filter cell. By late 1998, with degradation of the media, support gravel disruptions, and filter sand penetration into the underdrain, the maximum filtration rate had decreased to 1.5 mgd per filter, yielding a maximum tertiary treatment process flow rate of 24 mgd. The average length of a single mixed media filter run was 8 hr during the summer months and 12 hr during the remainder of the year. At times the mixed media filters experienced even shorter filter run times because the filters became clogged, especially during wet weather.

By the end of the Demonstration Project in the fall of 1998, Tonawanda was beginning to reap the benefits of its willingness to pioneer this technology. Vast improvements in many areas were being seen and quantified, including:


 

Backwash water volume reduction    Monomedia significantly reduces the amount of the backwash water volume that is recycled to the head of the plant for re-treatment. This reduction is attributed to:
— Reduction in the peak backwash flow rate [from 20 to 7 gal/min)/ft²] and total volume of water used during the backwash cycle
— Increase in the filter run times from 8 to 24 hr during the summer and from 12 to 48 hr during the remainder of the year
— Reduction in the number of filters that are normally operating from 16 to 12.

Filter system gal/ backwash per filter gal/day per filter Active filter cells mgd
Mixed media 82,000 192,000 16 3.07
Monomedia 70,000 53,000 12 0.64
      Net water volume reduction:
 
2.43


 

    These figures show that 2.43 mgd of wastewater will not be pumped through the filters for backwashing and will not be re-treated through the plant as a result of the decreased backwash water use with monomedia filtration at the Tonawanda Plant.
 

Energy and economic savings    The Tonawanda Treatment Facility has historically chlorinated the backwash water for the mixed media filters and continues to chlorinate the backwash water for monomedia filters. Chlorine helps to minimize biological growth in the underdrain block, which has very small air release holes in the primary chamber that can easily become plugged. Since the addition of chlorine is proportional to the amount of backwash water used, a reduction in the amount of backwash water has resulted in a reduction in the amount of chlorine required. This has reduced chlorine use from 182,000 lb/yr to 38,000 lb/yr, resulting in a net cost savings to the Town of $18,800.
 
The Town has realized additional savings because it uses less energy for backwashing. This has been achieved because of a decrease from 125 hp to 50 hp backwash pumps, a decrease in the frequency of backwashes, and a reduction in the amount of recycled backwash water that is re-pumped at the head of the plant. This has resulted in net energy savings of 444,600 kWh/yr. There is also a reduction in the peak electrical demand at the plant because of the pumps reduced horsepower. In all, the plant is now realizing a total energy cost savings of $30,300/yr.
 

Wet weather treatment capacity    The greatest benefit seen at the Tonawanda facility has been the vast improvement in the wet weather treatment capacity of the plant. Conventional fine-grained mixed-media filters are usually designed for a peak filtration rate of 4 to 5 (gal/min)/ft² but often operate at significantly lower rates because of media breakdown, surface blinding, and so forth. The Tonawanda plant has been able to operate continuously its new monomedia filters at flow rates of more than 7.5 (gal/min)/ft² for extended times, with backwash frequencies of 24 hr or longer. This has resulted in a more than 150% increase in Tonawanda's wet-weather tertiary treatment capacity, from 24 mgd to more than 64 mgd.
 

Effluent quality    To analyze further the effluent quality produced, several parameters from 1997-1998 were compared with to data from May-September 1999 when the monomedia filters were in full operation. (The parameters were plant influent suspended solids, plant effluent suspended solids, the daily average plant flow, and the daily instantaneous maximum plant flow.) See Figure 4 which shows a comparison of the Average Percent Solids Removal with conventional and with monomedia filters. The data indicate that the performance of the monomedia filters is similar to that of the conventional media and more than adequate to meet the treatment plant discharge requirements.

Figure 4. Average percent solids removal.

In addition, total suspended solids removal through the treatment plant was evaluated to compare effluent quality before and after the installation of the new filters. See Figure 5 which demonstrates that the switch to monomedia has fully maintained the quality of plant effluent.

Figure 5. Average effluent suspended solids.

Niagara County's experience

The Niagara County Sewer District (NCSD) completed the conversion of their facility to coarse monomedia filtration in late 1999. The improvements to plant wet weather flow treatment capacity have been as dramatic as those experienced at Tonawanda. The Niagara County plant, which previously used mixed fine-media filters with water-only backwash, opted for the wholesale replacement of the underdrain, support gravel, and media. The District installed the "T-Block" underdrain system, manufactured by Tetra Technologies, that accommodates the addition of air scour and is less prone to biological fouling. The District's filters have been demonstrated to handle more than 8 (gal/min)/ft² on a continuous basis. "What used to be a weak link in our treatment process is now one of the strongest assets in our plant," says NCSD's Frank Nerone.

So impressive were the results of the conversion to coarse monomedia at the Tonawanda and Niagara plants that the Erie County Southtown's Sewage Treatment Agency has authorized R&D Engineering to design the conversion of its 16-mgd treatment facility in Lackawanna, immediately south of Buffalo. The facility's current filter system, installed in the early 1990s, has degraded to average filter run times of 9 hr with total filtration capacity of 14 mgd, or 2.2 (gal/min)/ft². Construction of the conversion to coarse monomedia is scheduled for early 2001.

What's next?

One of the foremost challenges facing publicly and privately operated wastewater systems in the next several years will be to comply with regulations governing combined sewer overflow (CSO) and sanitary sewer overflow (SSO) treatment under USEPA policy initiatives. Most systems in New York State have begun identifying and quantifying CSOs and SSOs, and decisions regarding economical methods for minimizing and treating CSO-SSO flows will have a major economic effect on most sewered communities in the State.

Coarse monomedia filtration holds the promise to eliminate tertiary treatment as the wet weather flow bottleneck in many plants. In fact, for many plants the conversion to coarse monomedia may create excess filtration capacity beyond the flows that secondary treatment units can process during wet weather events. Can this excess capacity be productively used to improve the quality of primary effluent from flow equalization or overflow retention facilities? Should excess filtration capacity be specifically constructed for this purpose? Does coarse monomedia hold the promise of being an economical unit process for the treatment of CSO-SSO flows at remote locations in wastewater collection systems?

R&D Engineering is currently working with the Town of Tonawanda and NYSERDA to study coarse monomedia in the filtration of primary effluent to attempt to answer these and related questions. The potential effect on the wastewater industry will be truly significant if the results of this evaluation approach the outcome of the Tonawanda demonstration project. Is it any wonder that ever since the preliminary pilot study back in early 1998, John Camilleri has affectionately referred to coarse monomedia as "magic sand"?

Wastewater systems throughout the State will be watching closely as the Town of Tonawanda, Niagara County, and Erie County work to perfect this new technology in its application to standard tertiary treatment. Its deployment in alleviating effluent quality deficiencies in CSO-SSO flows and wet weather events is very promising, and all three western New York facilities will be leading the effort to capitalize on the economic and environmental benefits it will offer to municipalities across the State.
____________
Bryan T. Smith, PE is senior vice president and Karen M. Miller is director of marketing for R&D Engineering in Buffalo.


 

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