Back to the Basics

A new biostimulant economically treats wastewater sludge by breaking it down into gas, ash, and water

Despite the very best in design, there does come a time in every wastewater lagoon system when sludge must be removed.

To properly maintain and manage a wastewater lagoon system it is essential to know how much sludge exists and how quickly it is accumulating because lagoon sludge can lead to the following problems:

  • Ammonia and phosphorous feedback resulting in permit violations;
  • Fecal coliform violations;
  • Biological Oxygen Demand and Total Suspended Solids permit violations;
  • Odor complaints;
  • Algal blooms and aquatic weed problems;
  • Reduced capacity leading to short-circuiting; and
  • Diffuser clogging or buried aerator electrical cables.

A sludge blanket becomes a problem by releasing ammonia, organic acids, nutrients, and other anaerobically generated byproducts back into the water column. The release of stored nutrients by a sludge blanket is known as benthal feedback or benthal release.

Persistent algae blooms are one sign it may be time to remove sludge. Because recurring algal blooms are closely associated benthal feedback, the state of Vermont1 recommends removing sludge after 18 inches have accumulated.

Algal blooms not only create total suspended solids (TSS) problems, but they also contribute to 5-day biological oxygen demand (BOD5) violations. Because algae consume oxygen under dark conditions, excess algae in the BOD5 test bottle can inflate effluent BOD5 test results. "In 24 Colorado municipal wastewater treatment lagoons, two-thirds of the effluent BOD5 violations were due to algae overgrowth."2

Accumulated sludge can lead to recurring algae blooms. Not only can nutrients released from the sludge blanket feed algae and bacteria, but sludge solids themselves can slough off into the effluent and cause high TSS and BOD.

Solving these problems by removing sludge is expensive. Average sludge removal costs across the nation are about $.07 per gallon, or about $100 per dry ton.3 This makes sludge removal from lagoons very expensive -- especially for small communities. It is not uncommon to see a dredging quote of $300,000 for a 30-year-old, 6 million gallon lagoon half-full of sludge. Some communities may find it difficult to raise this much money, especially if the state is demanding quick action to remedy BOD5 or TSS violations resulting from the nutrient feedback of accumulated sludge.

Planning For the Future

The eventuality of lagoon sludge removal costs must be considered early, before problems develop. Communities should set money aside each year in a fund to cover the often large expense of lagoon sludge removal and disposal.

Cities and businesses have several different options for removing sludge from their lagoons. Most options involve removing the lagoon from service and hauling the sludge off site. Operators can drain their lagoons, dry the sludge in place, and eventually haul it off for land application or placement in landfills. Sludge can also be removed by floating a dredge in the water and pumping the sludge out to a pit, drying bed, belt press, or directly to land application equipment for subsurface injection. These options can be problematic and quite expensive.

Some of the country's best lagoon operators have discovered a cheaper way to remove lagoon sludge in place by using a biochemical product from Probiotic Solutions.

The technology behind this chemical product was first used in the agricultural industry to accelerate stubble digestion and crop residue management in farmers' fields. These "probiotics" have also been used to accelerate the feed uptake rate in the digestion systems of livestock.

In a farmer's field, these probiotics stimulate the bacteria, protozoa, and fungi already in the soil to oxidize crop residue (stubble) faster and more completely. In the digestion system of an animal, probiotics leverage the work of existing bacteria to break down feed rations faster and more completely for greater milk production or animal weight gain. This allows the farmer or producer to get greater yields from his livestock and more value from the feed the animals are eating.

No Interruptions

When used in wastewater lagoons, probiotics accelerate the destruction of sludge while the system remains online; operators add BIO ENERGIZER, a biostimulant, to accelerate the digestion of organic matter in their sludge.

The biostimulant is not a bacterial inoculant, but a biochemical product that affects the cell wall permeability and the cellular metabolism of the existing bacteria, fungi, and protozoa in a wastewater lagoon system. The benefit of biostimulant is that it can be used to break down sludge while the lagoon stays online. There is no inconvenient down-time, no sludge handling hassles, no odors, and the biostimulant is inexpensive relative to the cost of mechanically removing and handling the sludge. In fact, it is typically about one-tenth the cost of mechanically removing and disposing of the sludge.

The biostimulant eliminates expensive dredging costs by accelerating the conversion of sludge to carbon dioxide and water through a "wet burn" process. This removal process is similar to what happens when a 50-pound log is burned in a fireplace. After the fire burns out, what is left over? How much does the log weigh now? Where did the log go?

As the log burned, its mass was converted to gas and ash. The organic matter in sludge undergoes a similar process at the bottom of a wastewater lagoon. In a lagoon sludge blanket this "wet burn" process is known as bio-oxidation, where sludge is oxidized by microbes into gas and water.

Accelerating the Conversion of Sludge to Gas and Water

Sludge consists of dead bacteria and algae bodies. Bacteria and algae are composed of carbon, hydrogen, oxygen, and nitrogen. During the oxidation of sludge by bacteria, these elements are converted into gasses and non gasses like methane, carbon dioxide, nitrogen gas, hydrogen sulfide, and water. So, when you think about it, sludge, like the log, is potentially nothing more than gas and ash.

Because the biostimulant accelerates the conversion of sludge to gas, ash, and water, the lagoons remain online. There is no draining, drying, excavating, or down-time.

Oxidizing sludge in place with the biostimulant is not a quick fix, nor does it reduce inorganic materials like sand, grit, gravel, or clay. Operators can expect to see an average of 2 inches of sludge reduction per month. This reduction includes the oxidation of existing sludge and oxidation of the new load entering the lagoon daily.

In May of 2003, the City of LeRoy, Ill., decided to desludge their primary lagoons using BIO ENERGIZER. Accumulated sludge was taking up valuable lagoon capacity and causing odors and high-effluent BODs. LeRoy's lagoons serve a population of 3,300 people, plus businesses who contribute about 700,000 gallons of wastewater each day to the 26-year-old, 2-celled lagoon system. Sludge accumulations in some areas approached 7.5 feet, and too little water existed over the sludge blanket to maintain odor control. After 14 months of probiotic treatment, about 70 percent of the sludge was removed from the pond.

Scott Bryant, chief operator at the City of LeRoy had this to say about the biostimulant used in the cleanup: "Prior to using BIO ENERGIZER, half of my lagoon capacity was filled with sludge. After 2 years, my average sludge blanket depth is now down to about 12 inches. I highly recommend this product to anyone."

This product also works well with difficult-to-digest industrial waste. For example, the City of Lemoore, Calif.'s 4-cell wastewater lagoon system has been receiving about 2.7 million gallons of domestic and industrial wastewater each day. Lemoore's industrial wastewater is composed of whey, disinfectants, cleaners, and other difficult-to-digest cheese manufacturing waste. After many years of operation, the sludge blanket reached 9 feet, breaching the surface of the pond. Concerned about the loss of capacity, the city began looking for ways to remove the sludge from the pond.

Dredging, drying, and removal costs were estimated at about $341 per dry ton. The pond contained about 3,502 dry tons of sludge, which brought estimated removal costs to about $1,194,000. To keep the lagoon online, and to save money, the city decided to use BIO ENERGIZER to oxidize the sludge in place. After 2 years, the biostimulant removed 1,268 dry tons of sludge, for a savings of $321,388. It is also impressive to note that this sludge blanket reduction was accomplished while the pond remained online and continued to receive its normal daily load. The biostimulant not only removed 1,268 tons of existing sludge, but for two years it destroyed all the new sludge added to it for an average net sludge blanket reduction of more than 2 feet.

When the biostimulant is added to lagoons treating 100 percent domestic waste, sludge destruction results are typically faster and more complete than those seen in Lemoore's industrial/domestic lagoons. Results treating 100 percent domestic waste like in the case of Brook, Ind., are typical. The operators in Brook were struggling to keep their pond system in compliance and were facing enforcement action from the Indiana Department of Environmental Management for CBOD and TSS violations. Brook operates a 2-cell system that treats about 77,000 gallons each day and serves a population of 880 people.

A routine measurement of the sludge blanket showed an average of 2 feet of accumulated sludge. An operating depth of just less than 5 feet meant the sludge blanket occupied about one-half of the pond volume. Engineers working with the city of Brook decided to use BIO ENERGIZER to remove the sludge while the lagoons stayed online. After seven months of treatment with the biostimulant, about 1.5 feet of sludge was removed from the pond. As the lagoon was sludge judged at the end of the seven months, operators noticed how easily the sludge judge cut through the water and remaining sludge blanket without being forced. The operators reported how the sludge judge now just "bounced" on the bottom of the pond. Core samples also revealed little to no odor.

Because the processes occurring at the bottom of a shallow lagoon are similar to the processes occurring in an aerobic digester, an aerobic digester is a great place to closely evaluate the biostimulant's destructive effectiveness. In aerobic digesters, sludge volume and mass are measured daily, so, when the biostimulant is added to an aerobic digester, conversion of solids to gas and water is easily measured and observed.

References:

  1. Olander, Paul, (August 1993) "Troubleshooting/Optimization of Aerated Lagoons," Vermont Department of Environmental Conservation, Public Facilities Division.
  2. Richard, M.G. and Bowman, R. (1991) "Troubleshooting the Aerated and Facultative Waste Treatment Lagoon," presented at EPA's Natural/Constructed Wetlands Treatment Systems Workshop, Denver, Colo.
  3. Probiotic Solutions, (2003) "Survey of Systems Recently Dredged," In-house research by Probiotic Solutions.

This article originally appeared in the 09/01/2005 issue of Environmental Protection.

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