Many companies are manufacturing synthetic packing materials for retrofit in existing activated sludge systems. The packing may be suspended within the mixed liquor or fixed in the aeration basin In either form, the addition of fixed film support allows for a greater biomass concentration in the biological unit which allows for higher waste treatment loadings with the same size basin by keeping consistent F/M and solids flux in the secondary clarifiers within design operating range.
As with all fixed film systems, the basis for performance is the establishment of a stable biofilm on the carrier. As with the trickling filter, the biofilm works in the same manner.
One of earliest retrofit suspended growth systems utilized polyurethane foam pads with a bulk density of 0.95 (or slightly lower than water). Adding the pads at 20 - 30 by basin volume with screens to keep the pads in the aeration basin, results in an equivalent MLSS concentration of 5,000 - 9,000 mg/L. Note that this system maintains the biomass recycle from the secondary clarifier.
Another retrofit, is the moving-bed biofilm reactor modification where the aeration basin is filled with 25 - 5i0% tank volume of polyethylene solid carrier media. The media and associated biofilm are suspended by aeration and maintained in the tank by a screen leading to the clarifier. What is good about the MBBR system retrofit, is the secondary clarifier does not recycle biosolids to the aeration basin - the clarifier only settles sloughed solids. Both oxygen and mixing are provided by coarse bubble difffusers that have much fewer problems with fouling when compared to fine bubble diffusers. Additional sections can be used for denitrification by replacing aeration with mechanical mixers to provide for media under anoxic/anaerobic conditions needed for nitrate/nitrite removal to nitrogen gas.
There have been many searches for information on red worms in wastewater treatment plants over the past few weeks. The worms are a common problem in warm weather and when the treatment plant has sufficient D.O. and is running well.
The worms are not really worms but the larvae of the midge fly. You may notice the non-biting midge flying above the plant around the time the worms appear.
While their ecology is interesting - most people just want to know how to control this bloom that eats the MLVSS and can cause TSS issues in the effluent.
Option 1 - Strike
Strike is an EPA approved insect growth regulator (IGR) is applied in relatively low concentrations and preempts the midge fly life cycle and brings about control.
Option 2 - Aquabac XT
A Bacillus thuringensis based biological control agent that works for controlling midge larvae. The are killed when they consumer the spores in the Bt liquid. Aquabac XT is manufactured by Becker Microbial Products and is available from a number of vendors. I have included a link the label for Aquabac XT from one of the online vendors. http://www.allprovector.com/labels-msds/Aquabac_XT_Label.pdf
Dosing and application information for both products is included on the labels and websites. They are both effective at controlling the red worm/midge larvae. I have used both successfully in activated sludge and fixed film systems. Hope the information helps.
Often when dealing with questions on a system, I am given a trade name for a system which is thought to be unique. Well, in most cases the trademarked or patented system is just a variation on one of the time tested formats that have proven effective for years. In the past decade, many fixed film type systems have been installed - no matter if you call them fluidized bed, IFAS, FAST, RBC (Rotating Biological Contact), or even a tricking filter - all of these systems work on the principal of a biofilm attaching to a surface to support a high population of microbes that are not as susceptible to washout as a suspended growth (standard activated sludge or lagoon) system. We are now seeing retrofits where media is being added to increase aeration basin capacity by increasing the M in the F/M ratio. Today, I want to break down the various types of fixed film systems so comparisons of various "new" technologies is more easily accomplished.
All fixed film system rely on a "host" surface for a microbial colony that forms a biofilm. The standard film is represented in the graph below and is found in all variations of fixed film systems.
I am going to do a series of posts on each of the fixed film system technologies with the goal being to give a general overview of the technology and most importantly the pros and cons of each. Today I am going to start with the oldest of the fixed film technologies - the trickling filter.
The oldest of the fixed film system is the trickling filter. The most basic systems spray influent over rocks or other media contained in a tower. Treatment capacity and efficiency can be mproved by adding blowers to provide additional oxygen to the lower levels of the tower. Existing trickling filters can also be improved by utilizing newer, higher surface area plastic media rather than rock or other traditional media. Other mostly industrial trickling filters, recycle biologial solids from the secondary clarifier to "re-seed" biomass to the front of the system.
Pros of Trickling Filters
Expected Treatment Efficiency
Loading (kg BOD/100 m3/day) BOD Removal %
Low Rate Filters 40 80 - 90%
Intermediate Rate 64 65 - 85%
High Rate 160 50 - 75%
Roughing Filters 480 40 - 65%
Non-filamentous bulking is often overlooked as a wastewater treatment problem. Operators facing viscous bulking often complain of "gelatinous" floc, that billows over the clarifier weir and is difficult to dewater on the secondary press.
We see viscous bulking when the bacteria begin to produce excessive quantities of extracelluar polysaccharides (EPS). In normal good floc, it is the EPS that acts as the glue to hold bacteria cells and adsorbed (outside cell wall) particles. When cells begin to overproduce the EPS, the previous glue begins to hold excessive amounts of water and is very vulnerable to sheer. The resulting sludge does not compact well and can easily be carried over secondary weirs. The return sludge (RAS) concentration is lower as we are returning more water relative to biological solids which further compounds the problem.
You can see viscous bulking under normal light microscopy as "fingers" and gelatinous appearance. The appearance can be enhanced for better observation by adding India Ink to the slide. The India Ink will not penetrate the EPS and will appear as clear zones around the floc.
Causes of Viscous Bulking
Ponds often develop surface films often develop in warmer months. Environmental conditions promoting film formation include warm weather, low flows, and sunlight promoting algae blooms. While a slight film may cause no problems it can promote conditions for odor complaints (H2S & volatile organic acids) and be a precursor to an algae bloom. In this post I will cover what compromises the film and how it arises in the pond.
While algae is important for normal pond activity. Excessive nutrients and summer temperatures can lead to a cyanobacteria bloom. These "algae" are actually a photosynthetic bacteria. They tend to form a bright green, blue-green surface scum and increase suspended solids in the pond. Besides causing problems with effluent TSS, cyanobacteria produce chemicals that contribute to odors and off-flavors in ponds. If the bloom is substantial, cyanobacteria can also prove deadly to wildlife. In 2013, an elk herd in New Mexico drank from a pond with a cyanobacteria bloom and was found dead in the immediate vicinity.
The second source of film is less obvious from cursory visual clues. In a quiescent lagoon with anaerobic activity often forms a film containing fatty acids. Since the fatty acids are naturally hydrophobic, we get in effect a "soap scum" on the surface. This film layer prevents normal oxygen transfer which further increases the size of the anaerobic zone. As a result, the pond often produces H2S and other malodorous compounds.
Lagoons are one of the oldest and most cost-effective ways to treat municipal wastewater. Lagoon treatment systems are often found in rural and small municipalities where enough land is readily available. Even with the latest generation of wastewater permits with BOD/COD, TSS, ammonia and phosphate removal required for needed effluent treatment; a lagoon system offers the easiest to operate at the lowest cost for many facilities and towns.
There are four types of lagoons seen in field practice:
Grease enters municipal wastewater treatment plants from multiple sources including restaurants, households, and light industrial customers. The fats, oils & grease (FOG) cause problems from the source point all the way down to the outfall of the sewage treatment plant. Each impact is detailed below
Upon allowing grease to enter a drain, the grease begins to accumulate on drain lines. With severe buildup, grease can clog the line causing backups and slow drains. Even with moderate grease accumulation, the restaurants will have problems with odors and drain flies which breed on the grease in the lines. Without proper grease trap maintenance, go the store will have exterior odors and experience excessive grease carryover into municipal sewer lines.
Neighborhoods, shopping centers, and other areas of development are increasingly required to build storm water retention ponds to prevent downstream flooding and pollution from leaving the immediate area before treatment. Being relatively shallow with little mixing, these ponds often have problems related to (1) algae blooms, (2) odors, (3) oil sheen, and (4) solids buildup.
If operators considered the ponds as a small biological treatment unit rather than a "wide spot in the ditch", the problems listed above can be remedied with ease.
Lagoons treating waste water or storm water eventually build up organic solids that reduce hydraulic retention time, periodically washout solids in the effluent, and cause odors from H2S production. The primary quick way to remove solids is to mechanically dredge, dewater and landfill these solids. However, this is a big ticket event that many facilities do not have the budget to undertake. Over the years numerous additives and technologies have been proposed to enable microbes to break down organic sludge into carbon dioxide, water, and methane. None has proven particularly effective and dredging has remained the primary way to deal with sludge.
I have been monitoring new technologies for "biodredging" or biological sludge decomposition for years and evaluated numerous new technologies. To be successful, biological dredging requires the following:
Anammox is one of the newest technologies for treating both ammonia and nitrite in wastewater. In the past operators running systems with long sludge ages noticed that nitrification did not consume as much oxygen and alkalinity as was calculated by normal Nitrosomonas and Nitrobacter ammonia oxidation. Furthermore, the denitrification process to remove nitrate/nitrite had less of both compounds entering the anoxic/anaerobic zone. What was happening?
Anaerobic Ammonia Oxidation (ANAMMOX) is pictured in the nitrogen cycle graphic at left. The process is as follows (forgive the lack of subscript):
NH4 + NO2 --> N2 + 2H2O
The microbes responsible for ANAMMOX conversion of ammonia and nitrite have been recently isolated and we are still in the process of understanding their microbiology in wastewater. Here is what we know:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
Click to set custom HTML