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WaterOperator.org Blog

Articles in support of small community water and wastewater operators.

Screens: An Important First Step in a Wastewater Treatment Plant

By Phil Vella

No matter what size wastewater treatment plant you have, screening equipment at the headworks is a necessary requirement. Screens or pretreatment devices are designed to remove or reduce large solids like wood, cloth, paper and plastics from the waste stream. This not only allows downstream treatment process to be more efficient but also protects the equipment such as pumps.

Several different types of equipment can be used to meet these objectives and there is no one-size-fits-all solution that can be applied to every headworks situation. Some of the limitations of small systems are low flows, space and financial considerations. The following will focus on those options most likely to fit into a small wastewater treatment plant. 

As with most equipment at a plant, screens come in a variety of sizes, capacity, automation and cost. In general screens may be classified as coarse, fine and micro and are based on the size of the screening openings. The discussion here will focus on course screen technology with openings 6 to 36 mm (0.25 to 1.5 in.).

Manual Bar Screens

With the limitations of small systems, a manual bar screen may be a great option. These screens have vertical bars approximately 1 to 2 inches apart to catch the incoming debris. Although very basic, they do provide a good level of protection for the plant. An example is shown in Figure 1. However, as with most basic equipment, there are limitations.


Source: Islamic University of Gaza.

The most obvious limitation is that this is a manual operation and requires dedicated manpower and can be a burden to small systems. This is especially true during high flow events such as storms that may require more frequent raking and may also create more of a safety hazard for the operator.

Automatic Bar Screens

To limit the labor involved with manual bar screens, there are several automated options available. These can be classified into different group types. Chain Driven Screens, Catenary Screens, Reciprocating Rakes, Continuous Belt Screen and many variations of them. A summary of different types of screens with their advantages and disadvantages is given in Table 1. These options also commonly use vertical bars to capture the solids and remove them with an automated raking system. Since these are automated, the cost and other operational costs must be considered. In addition, these systems are larger than the manual screen so adding this to a facility may require civil engineering to modify the influent channel or the headworks building if placed there. You may have reduced the labor cost but have increased the capital and infrastructure expense.

Although course screens can remove large material at the head works, disposal of this material becomes and added cost and requires operation and maintenance.  In addition, the wet screenings collected are smelly that can attract vermin and result in odor complaints from the community. 

TYPE OF SCREEN

ADVANTAGES

DISADVANTAGES

CHAIN OR CABLE DRIVEN SCREENS

Design in the market for many years
Simple channel construction
High screenings loading rate
Insensitive to Fat, Oil, and Grease (FOG)
Low headroom required

Submerged components subject to wear and tear

RECIPROCATING RAKE SCREENS

No critical submerged components
Widely used

Low screening loading rate
High overhead clearance, particularly at deep channels

CONTINUOUS SELF-CLEANING SCREENS

Medium to low headroom required
Allows a pivot design for servicing the unit above the channel

Several moving components
Components subject to wear and tear

ARC SCREENS

Simple design
Lower capital and operational cost
No drive parts under water Utilizes 100% of channel width

Limited to small to medium flow plants
Not suited for deep channels

CATENARY SCREENS

Simple to operate
Easy to maintain

Chains are very heavy and difficult to handle
Large footprint

Source: WEF, Manual of Practice 8, 2017

The following are examples of some of the screen options available to wastewater treatment plants.

Multi Rake Chain Driven Bar Screen (Automatic/Self Cleaning) Source 

Multi Rake Automatic System



Reciprocating (Single/Basket) Rakes | Source

Catenary Bar Screen | Source

Arc Screen | Source

In summary, there is no right equipment choice for all headworks screening circumstances. Individual factors such as flow rate, solids loading, cost and infrastructure modifications must be considered. Choosing the correct option is important not only in protecting downstream equipment but also for efficient and effective solids removal resulting in proper wastewater treatment. 

Featured Video: Lift Station 2 Pump Cleaning

Featured Video: Lift Station 2 Pump Cleaning

Lift stations are designed to move sewage from a lower to higher elevation through pumps and pipes. Once pumped to a higher elevation, sewage continues to move through the collection system via gravity to reach the wastewater treatment plant. Utilities typically install one of two types of lift stations. Submersible pump lift stations, also called wet well lift stations, contain the pumps, sewage, and floats all in one vault. Dry well lift stations use two vaults to separate the pump system and wastewater. To learn more about lift station operations, components, sizing calculations, inspections, maintenance, emergency response, and pump selection for both types, check out the U.S. EPA’s Wastewater Utility Operation and Management for Small Communities – Lift Station Overview webcast recording.

Learning the theory behind lift station operations, maintenance, and emergency response is an essential component to any wastewater operator’s training, however hands-on experience is also important. While we can’t give you hands on experience in a blog post, the following video offers a real world example of lift station maintenance. In this video, a Minnesota operator demonstrates how to remove debris from a clogged pump at his facility’s lift station. He also overviews the various components on the lift station control panel. Please note that when troubleshooting lift stations you should first be trained in operator safety. Safety topics to review before working with lift stations include lock out tag out, confined space, electrical safety, fall protection, crane safety, and tail gate safety.

Increasing Attention to Significant Noncompliance Dischargers

Increasing Attention to Significant Noncompliance Dischargers

WaterOperator.org would like to thank EPA’s Office of Enforcement and Compliance Assurance for helping develop this blog post as part of its outreach to permittees about the Clean Water Act National Compliance Initiative.

National Pollutant Discharge Elimination System (NPDES) permit compliance protects public health and the environment from the release of harmful contaminants. During FY 2018 approximately 20% of the nation's 46,000 permit holders were in significant non-compliance (SNC) violations. SNCs are designated as serious violations warranting enforcement response if not promptly resolved. These violations ranged from significant exceedances in effluent limits to reporting failures. To better defend environmental and public health, the U.S. Environmental Protection Agency (EPA) has initiated a National Compliance Initiative (NCI) for NPDES permits and, in September, released a Compliance Advisory.

The NCI uses a full range of compliance assurance tools to reduce NPDES permittees non-compliance. By FY 2022 the NCI aims to reduce SNC rates by half. Small systems, this includes you too! This NCI will target facilities of all sizes equally. More attention will be directed toward facilities approaching or already in SNC. Facilities failing to comply can be subject to increased monitoring, inspections, enforcement actions, and other compliance activities. The NCI notes that permittees that voluntarily disclose and correct violations may be eligible for a reduction or elimination of penalties.

While this initiative might feel intimidating, the NCI offers resources to those taking immediate compliance efforts. The EPA recommends permittees first assess compliance by reviewing discharge monitoring reports (DMRs) and the Enforcement & Compliance History Online (ECHO) tool. If your facility does require assistance, reach out to your NPDES permitting authority for assistance. Other organizations like Rural Community Assistance Partnership (RCAP) and National Rural Water Association (NRWA) may also be able to provide assistance. Each state implements their own NPDES programs with the exception of New Mexico, Massachusetts, New Hampshire, and areas within Indian Country which are managed federally.

Small system SNCs can originate from a variety of causes. Failure to monitor, analyze, and report wastewater samples according to your NPDES permit can lead to a violation. Alternatively, incomplete or inaccurate compliance data transferred from state systems to the EPA’s Integrated Compliance Information System (ICIS) system can result in inaccurate identification of SNC permittees. Checking your compliance status in ECHO can prevent these complications. In addition to monitoring and reporting violations, unplanned discharges such as from a sanitary sewer overflow can also result in SNC. When wastewater effluent exceeds NPDES pollutant levels, utilities will also fall out of compliance. These exceedances typically arise as a result of operational, design, or administrative issues. We recommend the following resources to help improve your NPDES compliance:

Activated Sludge Process Control and Troubleshooting Methodology
Resolve 95% of your activated sludge process control issues using this Ohio EPA manual.

EPA’s ECHO Electronic Tool
The ECHO Detailed Facility Report tool helps facilities monitor compliance and verify the cause of SNC. Learn how to use the tool through the “Intro to ECHO Webinar” and other materials on this webpage.

EPA Webinars: Technical Assistance for Publicly Owned Treatment Works (POTWs)
The EPA hosts free webinars once a month offering technical assistance to POTWs. Find these webinars on our national training calendar or at the link above.

Managing Small Domestic Wastewater Systems
This TCEQ guide helps utilities develop plans to maintain or achieve compliance. The guide includes compliance checklists and planning worksheets. For additional assistance, TCEQ has developed and referenced resources for troubleshooting anything from bacteria control to process control.

Why Is My Lagoon Not Meeting Effluent Limits?
This article from the November 2016 issue of The Kansas Lifeline summarizes how to troubleshoot lagoon effluent compliance issues.

To find solutions to more specific compliance challenges, check out the WaterOperator.org resource library and small systems blog posts.

Inspiring the Future of Women in Wastewater

Inspiring the Future of Women in Wastewater

Editor's Note: We would like to thank NYC Environmental Protection for permission to use this photo.

Despite such worthwhile career prospects, in 2018 women made up only 5.8%  of water and wastewater operators according to statistics by the U.S. Census Bureau. As the water workforce ages and experienced operators retire, the water and wastewater industry can benefit by recruiting more women into the field. Mutually so, inquisitive women with interests in protecting public health and sustaining our environment have much to receive from the opportunities available within the industry.

In the field of wastewater treatment specifically, professionals can exercise their curiosity in the sciences while building technical and mechanical skills. The wastewater operator career not only offers extensive opportunity for growth and advancement, but starting positions often pay well, sustain job security, and will provide on-the-job training. The duties of an operator are an essential public service that require knowledge of wastewater safety, math, chemistry, microbiology, treatment processes, and utility operations and maintenance. Those with a penchant for problem solving and mechanical skills will fare well in the field. Other skills women can develop as a wastewater operator involve communication, presentation, collaboration, and eventually, management.

In the Empowering Women Podcast, Christen Wood, wastewater operations administrator of Summit County Department Sanitary Sewer Services and three time participant of WEFTEC’s operations challenge (with two of her teams making it all the way to nationals), describes the “happy accidents” that allowed her to stumble upon the field. She explains why she continues to hold such passion for her position noting that work as a wastewater operator is a career path, not a job. Listen to Christen’s interview to get a better idea about the type of tasks involved in the day to day work of an operator and the significance of those tasks in public and environmental health.

Still not convinced? NYC Water offers an excellent summary of the benefits a wastewater career will offer to women interested in the field. If you get anything out of this video, we hope its that you start to consider how you can fit into the wastewater industry! Find more information about the experience of women in the water industry at the Words on Water Podcast’s Inspiring Women in Water podcast series. The same podcast produced a separate interview with Mel Butcher, an engineering consultant at Arcadis. Her interview discusses how challenges that women do face as minorities in the industry can be overcome through honest conversation. 

Workforce diversity leads to new ideas, innovation, and progress. Consider how you can bring your skills to wastewater treatment.

Opinion: Challenges Quantifying COVID-19 Cases Using Wastewater

Opinion: Challenges Quantifying COVID-19 Cases Using Wastewater

Editor's Note: The views expressed in this post are the sole opinion of the author and not those of WaterOperator.org, our sponsors, or the University of Illinois.

In the May 5, 2020 edition of the WaterOperator.org newsletter, we highlighted ongoing research that uses wastewater-based epidemiology to monitor the spread of SARS-CoV-2, the virus that causes COVID-19. Especially in locations where no confirmed cases have been identified, any samples positive for SARS-CoV-2 viral RNA implies that there are people infected in that community excreting it. For that purpose, wastewater monitoring shows real promise as an approach to early detection. By monitoring wastewater influent, scientists hope we can develop an advanced warning system for outbreaks.

There has been significant buzz about using wastewater to quantify the actual number of people infected within a given service area, but there are some issues with quantifying cases I want to discuss. In our newsletter we highlighted MIT research aiming to quantify the number of infected from a large area in Massachusetts. In that article, the researchers point to concerns about meeting the litmus test of sound science.

The wastewater system they studied had 450 confirmed cases at the time of sampling. Results from this monitoring suggested the number of people infected could be much higher. They estimated somewhere between 2,300 and 115,000 infected people. A range this wide does little to help planners or health officials prepare for what might be coming during a pandemic.

Quantifying the number of people infected with COVID-19 using wastewater samples requires a much more comprehensive data set that we cannot gather today in any cost effective way. Here are a few of the problems I see in quantifying the positive COVID-19 population within a given wastewater system:

  • Not everyone excretes the viral RNA:
    A recent study published March 30 in the American Journal of Gastroenterology found that some COVID-19 patients exhibit gastrointestinal symptoms, with those patients more likely to produce a positive stool test. In other words, COVID-19 positive patients may not have ANY viral RNA in their stool. How do we identify those people?
  • Wastewater varies throughout the day and throughout the week:
    The influent coming through a plant varies based on the discharges from the users. A lot of variables can affect wastewater characteristics at the specific time a sample is collected. The time of day, time of week, and even the time of year can affect the flow into a plant depending on the types of users in the system.
  • Every system has a variety of sources for their wastewater:
    What percentage of the wastewater is residential? Are there commercial or industrial facilities that are discharging to the community system? If so, how much, and what types of businesses? In some communities, commercial and industrial users could make up a significant portion of the wastewater treated. In a rural area, the regional hospital may be in a smaller community making it a significant source and contributor. Other communities could be almost completely residential.
  • Sampling time and frequency can skew the results:
    Sampling time matters, as do the number of samples collected. How do we decide what is representative? Once an hour? Once a day? Sampling may need to be continuous to really understand the variability.
  • Wastewater collection systems leak:
    Leaking can occur both ways. Some wastewater leaks into the environment through the collection system while, at other times, a high groundwater table may be leaking groundwater into the collection system. I looked at approximately 50 smaller systems in Illinois to compare the amount of wastewater discharge to the amount of groundwater they withdraw from drinking water wells. (You would expect the amount withdrawn from wells to be more than that treated at the wastewater plant because of consumptive use.) In many cases systems were treating more wastewater than the raw water being used for their community supply and, in some cases, it was 2-3 times a much. This would be significant factor when using any volumetric approach to evaluating COVID-19 sampling results.
  • We have no benchmark to compare results:
    Without having data for a number of communities where the total number of residents with active COVID-19 infection is known, there is no way to validate assumptions and calibrate estimates built into the method. This would not be possible without a consensus understanding about the rate of asymptomatic cases.

If researchers must accept such a high degree of uncertainty, how can this method ever be accurate or useful? Many factors would have to be considered to quantify the number of positive cases for a given community and these would be unique to the individual system. That said, these are not likely new considerations for the talented researchers working on this effort. 

In the future I hope an approach to accurately quantify an infected population using wastewater-based epidemiology becomes a reality. It would be a tremendous asset. In the meantime, however, I believe our focus should be on evaluating the pitfalls mentioned above and working toward technologies/protocols needed within a wastewater plant to reduce uncertainty and move us closer to our common goal of protecting public health.

A Look at Protozoa in Wastewater Treatment Systems

A Look at Protozoa in Wastewater Treatment Systems

Wastewater treatment is fundamentally a biological process. When influent enters the microbial ecosystem of a treatment plant, nutrient removal is accomplished through the consumption of organic matter by microorganisms. The bulk of all nutrient removal is performed by bacteria, however protozoa and metazoa balance these bacterial populations and offer insight into wastewater conditions. Operators who understand the varying roles of wastewater microbes and the conditions that favor their growth can foster an ecosystem that promotes optimal treatment. In this week’s blog post we will review the niche protozoa fill in wastewater systems to enhance monitoring efforts and inform process control.

Roughly four percent of a wastewater system’s microbial ecosystem is made up of protozoa. Protozoa are single celled microbes both larger in size than bacteria and more complex. The most common types of wastewater protozoa include amoeba, flagellates, and ciliates. By consuming free bacteria and small, unsettled floc, protozoa enhance the clarity of the final effluent. Observing protozoa populations under a microscope can also alert operators of treatment conditions and sludge age.

Amoeba are predominant under a young sludge age because they require high nutrient levels or low competition to grow. Under shock loads of biochemical oxygen demand (BOD), high concentrations of particulate matter, toxic conditions, or low dissolved oxygen (DO), amoeba can also dominate. The latter two conditions generally trigger the amoeba to develop a protective gelatinous shell that gives them an advantage over other microbes. Furthermore, their slow movement reduces oxygen demand required for growth and reproduction.

Flagellates are typically present under a young sludge age as well. Since flagellates compete poorly with bacteria for the same soluble nutrients, their growth is favored at the younger sludge age before bacteria have had a chance to populate. As such, a wastewater sample relatively high in flagellates can indicate high soluble nutrient levels also known as a high food to mass (F:M) ratio.

Ciliates are favored under a healthy sludge age. While they do not consume organic matter, they do feed on bacteria making them excellent indicators of healthy floc formation and useful clarifying agents. Without ciliates, bacteria and algae populations can grow out of control in the wastewater microbial ecosystem. Among the three types of ciliates common to wastewater, each group has different conditions under which their populations are favored.

Swimming ciliates start to form as flagellates disappear. They may experience a spike in population when levels of free bacteria are abundant for predation. If too many free bacteria are present, the ciliate population surge can ultimately result in a cloudy effluent. Crawling ciliates dominate when those free bacterial populations begin to stick together forming floc through a secreted slime layer. This slime layer is produced when dissolved nutrients become limited. Since swimming ciliates cannot readily pick off bacteria within the floc, crawling ciliates begin to out-compete them. As they feed on bacteria, crawling ciliates can improve flock structure. A more mature sludge age with reduced BOD allows stalked ciliates to compete with crawling ciliates. Stalked ciliates anchor themselves to floc using the cilia surrounding their mouth structure to create currents that draw in bacteria. Once their food levels have diminished significantly more, stalked ciliates begin to branch into colonial units to acquire food more efficiently. If sludge continues to age, stentors and vaginocola protozoa grow in abundance.

For more information on wastewater protozoa and how to monitor them, we’d like to recommend the following documents. These resources and others like them can be found using our online, resource library.

Bacteria Protozoa – Toni Glymph
The guide overviews basic wastewater microscopy, slide preparation, sample collection, and the microbiology of activated sludge plants.

Wastewater Microbiology & Process Control - Wisconsin Wastewater Operator’s Association
Learn the about microscopes, slide preparation, and the microorganisms found during wastewater treatment.

Protozoan Count – Toni Glymph
This guide describes how to sample protozoa for observation under the microscope.

Managing Dissolved Oxygen in Activated Sludge Plants

Managing Dissolved Oxygen in Activated Sludge Plants

Sustaining optimal dissolved oxygen levels in activated sludge plants is necessary for biological treatment of organic material and ammonia. While raw wastewater often contains some amounts of oxygen, aeration systems can increase dissolved oxygen (DO), mixing, and the suspension of microbes through mechanical agitation or diffused aeration. Aerobic microorganisms use this oxygen to breakdown organic waste into inorganic byproducts. The amount of dissolved oxygen consumed by microbes during biological treatment is referred to as biochemical oxygen demand (BOD). According to an article by Triplepoint Water, approximately 1.5 pounds of oxygen is consumed for every pound of BOD oxidized. To oxidize one pound of ammonia, that value increases to 4.57 pounds of oxygen. Most plants aim to maintain around 2 mg/L of DO which allows microbes contained within the center of floc to receive oxygen.

Wastewater operators should regularly monitor oxygen availability in the form of dissolved oxygen. Insufficient oxygen levels will allow aerobic and nitrifying microbes to die and floc to break up. At DO concentrations under 1 mg/L, the potential for filamentous growth increases. On the other end of the spectrum, too much oxygen increases power consumption and, at very high levels, inhibits settling. Research has estimated that aeration can use up to 45 to 75% of a treatment facility’s overall electricity use. With an online DO analyzer equipped to automated controls, the EPA reports that energy costs can be reduced by as much as 50%.

Where and when an operator samples for DO will be determined by the requirements written in the facility’s National Pollutant Discharge Elimination System (NPDES) permit and basic process control. To compare dissolved oxygen levels throughout the day, samples should be collected at the same location. The Ohio EPA’s Activated Sludge Process Control and Troubleshooting Chart Methodology recommends that systems sample within 1-2 feet of the surface water near the discharge of the aeration tank into the clarifier. By collecting multiple samples in the same location throughout the week, operators can reliably determine if DO concentrations are sufficient for treatment while developing a DO profile. In addition, measuring DO at multiple depths and locations in the aeration tank can help find dead spots.  

To supply adequate DO, the Ohio EPA manual includes how to determine blower runtime based on organic loading and system design. We should  still note that temperature, pressure, and salinity can all influence the solubility of oxygen. Additional sampling locations can include the raw wastewater, aerobic/ anaerobic digester, and final effluent. Final effluent with high dissolved oxygen can cause eutrophication in the receiving waters, however low DO can harm aquatic organisms. Some permits set a minimum DO level for effluent to ensure aquatic organisms have the necessary oxygen levels to sustain life.

While every technique and tool has its strengths and weaknesses, operators can measure DO through a Winkler Titration test (see Michigan DEQ Laboratory Training Manual pg.91), electrochemical sensor, or optic sensor. The two sensors mentioned can be purchased as portable handheld meters or stationary devices. For automated blower control and continuous sampling, an online sensor is used. For NPDES compliance monitoring, measurements must be taken through an EPA approved method at the frequency specified in the permit.

When using any DO sensor, the EPA’s Field Measurement of Dissolved Oxygen (SESDPROC-106) procedures require that the equipment be well maintained and operated per manufacturer instructions. Upon initial purchase, probes should be inspected, calibrated, and verified for accuracy. During each additional use the instrument should be calibrated and inspected again. The EPA recommends checking instrument calibration and linearity using at least three dissolved oxygen standards annually. All maintenance and sampling activities should be documented in a logbook per NPDES requirements. Any time a measurement is taken, the temperature of the water and any notable wastewater conditions should also be recorded in the logbook. 

Dissolved oxygen is a frequently monitored parameter in wastewater treatment systems. Operators should have a firm understanding of how dissolved oxygen is involved in wastewater processes and how they can manage DO to achieve compliance. Check out our online document library to find useful resources to learn more.

Featured Video: Wastewater Treatment Process Control Testing

Featured Video: Wastewater Treatment Process Control Testing

This week’s featured video was produced by the Athens Wastewater Treatment Plant. The plant serves a small town of approximately 1,050 people in West Virginia. In an effort to educate their small town and others across the country, Athens WWTP has developed a series of videos. In this particular recording, the plant will demonstrate several process control tests they use to evaluate their wastewater conditions. You’ll learn how Athens performs a settleometer test and monitors pH, temperature, dissolved oxygen, oxygen reduction potential, mixed liquor suspended solids, and volatile suspended solids.

Tests likes these are valuable for troubleshooting the dynamic environment of wastewater treatment processes and meeting regulatory compliance. As such, it’s important for sampling to be performed accurately, consistently, and in a location that is representative of the wastewater quality as a whole. The types of tests you perform, the number of samples taken, and the laboratory methods used to analyze these samples will depend on your system’s treatment type, chemical usage, equipment, and raw water quality. Results from the analysis will promote process optimization. A detailed copy of your facility’s sampling and testing procedures should be accessible in the utility Operations and Maintenance Manual for reference.

To provide more information on process monitoring, we’d also like to recommend:


Peracetic Acid (PAA) in Wastewater Disinfection

Peracetic Acid (PAA) in Wastewater Disinfection

Peracetic acid (PAA) has grown in popularity over the last several years for its use in the disinfection of wastewater and stormwater. Utilities use disinfectants as the primary mechanism to inactivate and destroy pathogenic organisms that spread waterborne disease. An appropriate disinfectant will sufficiently treat any disease-causing microbes including bacteria, spores, helminthes, and protozoa. While PAA technology has been employed in Canada and Europe for the last 30 to 40 years, this disinfectant has only become noticed in U.S. municipal wastewater treatment within the last 10 years. Competing with chlorine, an already well-established disinfectant, its use is still slow growing, however systems are discovering that PAA offers several benefits to wastewater treatment that chlorination does not.

What is peracetic acid? The alternative disinfectant is a clear, organic peroxide compound that readily hydrolyzes to acetic acid and hydrogen peroxide in water. It’s characterized as a strong oxidant and fast reacting disinfectant. Commercially available peracetic (CH3CO3H) is purchased in an equilibrium mixture of acetic acid (H3CO2H), hydrogen peroxide (H2O2), and water (H2O). Manufacturers typically add a stabilizer as well. The following formula represents the equilibrium equation: CH3CO2H + H2O2 ←→ CH3CO3H + H2O.

PAA can generally be purchased in concentrations of 5% to 22%. When PAA decomposes in water, free hydrogen peroxyl (HO2) and hydroxyl (OH) radicals are formed. These radicals have significant oxidizing capacity that play an active role in microbial disinfection. According to the EPA, bacteria are destroyed through cell wall lysis and leakage of any cellular constituents.

Wastewater systems consider moving to peracetic acid for several reasons. Unlike chlorine, PPA decomposes into biodegradable residuals of vinegar (acetic acid) and hydrogen peroxide that can pass fish toxicity tests without removal. These residuals are not toxic, mutagenic, or carcinogenic. Bioaccumulation in aquatic organisms is also highly unlikely. Neither chlorinated compounds nor harmful disinfection by-products (DBPs) are produced with its use. As such, PAA has been considered the potential answer to tough DBP regulations. Peracetic acid can also disinfect over a wide range of pH and is unaffected by nitrate and ammonia concentrations.

Chemical handling of PPA is toted for being easier and safer than chlorination. The disinfectant can be stored for long periods of time exhibiting less than 1% decrease in activity per year when properly stored. Its use does not require any special risk management plans (RMPs) required by the EPA when handling certain toxic chemicals. For systems that operate under cooler conditions to prevent contamination or elevated temperatures, PAA has a low freezing point. Switching to PAA requires minimal retrofitting with the chemical itself being offered at prices competitive to other disinfectants.

There can be some disadvantages to peracetic acid. Depending on the formula purchased, PAA introduces varying amounts of acetic acid into the wastewater effluent. This can contribute to biological oxygen demand (BOD) and may not be appropriate for systems that are struggling to meet these limits. The biggest challenge wastewater systems face is regulatory approval. While PAA has been approved by the EPA as a primary disinfectant, each state regulatory agency must also approve its use. A WaterOnline guest column includes an infographic of states that have approved PPA as of 2017. The guest column discusses how systems can approach local regulatory agencies to seek approval on a case-by-case basis.

The overall effectivity of PPA will depend on wastewater characteristics, the PAA concentration, contact time, and the reactor configuration. Dosage will depend on the target organisms, wastewater quality, and level of inactivation required. When monitoring PAA residuals, operators can use the same analyzer and method as for chlorine residuals. A standard EPA sampling method does not yet exist. The lack of established methods and protocols for PAA makes approval difficult for local regulatory agencies. To help investigate the use and implications of PAA in wastewater, the Water Research Foundation (WRF) completed a study to evaluate effluent toxicity as well as dosage and contact times required to meet compliance. Metro Vancouver’s Northwest Langley WWTP in Canada has also published findings from a multi-year pilot program that used PAA as a disinfectant. More studies will have to expand on existing research until peracetic acid can become easily and widely adopted.

Using Reed Beds for Sludge Treatment

Using Reed Beds for Sludge Treatment

The use of reed beds in both central and decentralized wastewater treatment systems can offer a low cost and energy efficient opportunity to process sludge. Originally developed in Germany, the practice was brought to the United States in the 1980s. Under this technology, a variety of marsh grass, also known as Phragmites, is planted in reed beds built with concrete walls and lined with an impermeable layer to protect groundwater. TPO magazine suggests using a concrete bottom because PVC liner can be easily damaged during maintenance. The beds themselves contain a porous, finely aggregated media such as sand or recycled glass (pg. 12). This media allows the reeds to grow and excess liquid to pass through an underdrain system connected to the head of the plant for recycling. Risers can help distribute and load the sludge.

After the reeds have been established during a period of roughly three months, sludge can be loaded into the beds every three weeks. As the plants’ extensive root structure absorbs sludge moisture, water will be released through leaves and into the atmosphere via evapotranspiration. The microbes found in the root rhizome will help the sludge continue to break down. During the winter months when the reeds are dormant, the freeze-thaw cycle will allow liquid to easily separate from sludge to continue dewatering. When spring arrives, the reeds will return to their active growing cycle.

According to TPO Magazine, reed beds can adequately manage facilities that treat up to two million gallons per day provided that the required land is available. The reeds themselves can handle climates that experience several weeks of freezing temperatures during the winter. Before temperatures drop too low, operators will typically burn off the reeds in the fall. Alternatively, the reeds can be composted or disposed in a landfill. After approximately 8 years, the solids must be removed. At this time, the beds will be taken out of service in the summer and given an additional 90 days to dry out. Once the sludge is removed, the reeds will need to be re-established. A presentation by the Constructed Wetland Group provides a detailed overview of how to perform maintenance on reed beds.

While this technology is low maintenance and energy efficient, there are still pros and cons. As an advantage, reed beds can help to remove heavy metals from sludge. This should be considered during reed harvesting. As a drawback, constructing new beds requires significant capital costs, however utilities may be able to convert existing sand pits or drying beds to reduce costs. TPO Magazine notes that unpleasant odors can emerge during the spring when winter ice melts. Many scientists also worry that wastewater facilities using non-native grasses can encourage the establishment of invasive species. Phragmites spread predominantly through their underground rhizomes, laterally growing stems with roots. Furthermore, when non-native grasses escape into a new area, they can easily take over since their native competitors aren’t present. Facilities should practice careful harvesting and monitor the integrity of their bed structures to ensure containment. Despite these drawbacks, reed bed systems can be a successful and efficient form of sludge treatment even in comparison to conventional treatment methods.