Trickling Filter/Solids Contact Process

The trickling filter/solids contact process consists of three major process components:
1) trickling filters that produce a biological growth of organism that absorbs and utilizes much of the suspended colloidal and dissolved organic matter in the wastewater,
2) aeration basins that provide the means to flocculate the solids from the biofilters, and
3) secondary clarifiers that have flocculation wells in them that further flocculate the solids before they settle in the clarifier.

Operationally, treat them like you would any trickling filter. They will be removing the majority of the soluble BOD, converting/decomposing solids as would any biofilter. It is quite important that you maximize your primary treatment, as the pass through of solids can adversely affect your biofilter performance.

It is not the purpose of this operator notebook chapter to give instruction on the principles of trickling filter design, media types, operation, and maintenance concern, but to focus on and define the trickling filter-solids contact process as a whole. Please refer to the trickling filter chapters for more specific information on the biofilters themselves.
As we have learned, the percentage of BOD converted into microbes in the trickling filters is:
a) directly related to the amount of time that the wastewater is in contact with the zoogleal film in the biotower,
b) the temperature of the wastewater, (and in some cases the ambient air affecting the temperature of the wastewater)
c) the treatability of the wastewater, and
d) the type of BOD/COD and its treatability,

We also know that a minimum wastewater flow rate over the media is required to maintain an optimum environment for the zoogleal film to be effective. The modern plastic media has a far greater amount of surface area per square foot volume, and as such, requires a higher flow rate to wet it. We must also insure that an optimal amount of oxygen is available in the media for the microbes to maintain their aerobic metabolism.

The major purpose of the aeration tank(s) is to improve clarification of the trickling filter effluent. Traditional trickling filter plants produce an effluent that has about 45 mg/l BOD and 45 mg/l TSS. This is due to the fine, almost colloidal nature of the microorganisms and the partially decomposed organic matter coming in off the filters. Couple this with obsolete or poorly designed secondary clarifiers, and we have a lot of bugs going out to the receiving waters. The trickling filter-solids contact process has been shown to produce very consistent 30/30 effluents, if they are employing well designed, center flocculating clarifiers.

By returning "activated sludge" to the aeration basins from the secondary clarifiers, we raise the MLSS in the aeration basins to between 600 to 2,500 mg/l, (other references state 1,000 to 3,000 mg/L) and aerate the mixed liquor for about 15 to 20 minutes. In the facility I had the honor of working at, we had a detention time of about 40 minutes. This action forms larger biological floc that entraps the smaller solids and improves the settling qualities of the solids. Pilot studies, and trials have shown as little as 5 to 10 minutes accomplishes a great deal. During this time many plants do not see an appreciable reduction in the soluble BOD remaining in the trickling filter effluent, (even though one pilot study I know of showed a 33% reduction occurring in the aeration basins). I have seen other pilot studies, and visited plants that have achieved appreciable reductions in the soluable BOD. I expect this could be a function of the lighter organic loading at startup, as compared to many years later as the growth in the community creates more homes and businesses which in turn create higher loadings on the process. With the higher organic loadings there may be more opportunity for BOD reductions. Dissolved oxygen levels are normally held between 2-3 mg/l at the effluent of the aeration basins, with no portion of the tank below 0.5 mg/l DO. Oxygen levels above this are a waste of energy and may also prevent the formation of a good settling floc due to the turbulence resulting from that much air being applied to the basins.
The Return Activated Sludge may be returned either on a constant flow or flow proportional basis. With flow proportional method, the RAS is matched with the trickling filter effluent, which yields higher than normal sludge depth in the clarifiers during low flow periods such as on graveyards. With the constant flow method, the decrease in the treatment flow through the plant, allows for the activated sludge to be "stockpiled" in the aeration basins during the low flow periods, under aeration, rather than in the bottoms of the clarifier. But after watching an activated sludge plant shut down every day for 6 hours with no apparent adverse effects, I'm not so sure that this is all that relevant. You do consume more oxygen during graves in the constant speed mode as expected, due to the increase in microbes under aeration.

Mean Cell Residence Time (MCRT) or Solids Retention Time (SRT) as you all know, this is the calculation you perform to determine the length of time your solids are in the system. TF/SC plants use very short times, one day or less. The terms MCRT and SRT are starting to get "fuzzy." In many operator's minds SRT is for use when the secondary clarifier solids are NOT included in the math calculation, as shown here.

Process Flow Rate 13 MGD 13 mg/L secondary effluent TSS
0.033 MGD WAS Flow Rate 6,050 mg/L WAS concentration
2 aeration basins, capacity of 185,000 gallons each with 1,000 mg/L MLSS

<solids under aeration>
(Volume under aeration, MG)(8.34lb/gal)(MLVSS, mg/L) = SRT or MCRT
(Flow, MGD)(8.34)(EFF TSS, mg/l)+(WAS Flow,MGD)(8.34)(
<pounds suspended solids in the plant effluent> + < pounds sludge wasted>


(2 basins)(0.185 MG each basin)(8.34lb/gal)(1,000 mg/L SS) = 1 Day
(13 MGD)(8.34lb/gal)(13 mg/l)+(O.033MGD)(8.34lb/gal)(6050mg/1)

Please note that in the above calculation, that I am NOT including the solids in the secondary clarifier. You MAY do this ""if"" you do NOT carry an inventory of solids in the secondary clarifier. If you carry a solids inventory in the clarifier you should include this in your solids inventory.

It has been our experience in operating a trickling filter-solids contact process that the conversion of soluble organics and remaining TSS from the primary treatment process to biomass is indicative of biofilter performance, but that the removal of the biomass from the waste stream is a function of the aeration and clarifier performance.

ONE FOOT RULE ... many operators maintain a sludge blanket of one foot in the bottom of each secondary clarifier, which is completely picked up on each rotation of the sludge withdrawal arm and is utilized as either WAS or RAS. You do not therefore have a solids inventory to keep track of in the clarifiers.. and is NOT taken into account when you figure out your MCRT. IF YOU leave solids behind after the rotating arm passes, THEN you should account for these by adding them into the equation.

PEAK FLOW SOLIDS WASHOUT...for most plants, during our peak flow rates we will be losing more solids than we did on low flows. We may wish to enter into the reaeration mode to lessen the solids loading rates on the clarifiers during high flows.

Remember also, we tend to trap and therefore save, very fine solids in the Mixed Liquor floc, which then all settles out in the clarifier. A combination of higher flow rates and higher aeration rates are not conducive to bioflocculation, and as such, you may experience higher solids losses at lower than our typical "high flow rate" conditions.

During normal operation the filters continuously slough off the excess biological film. Most plants are also subjected to a spring and a fall "slough-off", but nothing compared to your "START-UP MAJOR SLOUGH". I had never read anything about this, nor have I since, and I am not sure why! It could not have happened to only us!

You will never have enough capacity in your digesters, thickeners, etc., to handle this event! Back in 1988 I was the Assistant Superintendent of a wastewater facility that had just undergone a major change in secondary treatment from the failed physical-chemical process type to the trickling filter-solids contact process. We started up the secondary treatment process, on March 17, 1988. We were "feeding the biofilters" about 70 mg/l of TSS, and a BOD of 120 mg/l. We had 20 mg/l TSS in our plant effluent, and a BOD of 25 mg/l after 7 days of operation. It was amazing, meeting all our permit requirements from day one! Then the biofilters started to smell like souring bacon grease. The odors grew worse with each passing day. We had the biofilter forced ventilation fans all going, the recirculation flow rate up. We were doing everything correctly. It became obvious that we were placing tons of organic matter into the towers with only a fraction of the required number of microorganisms necessary to decompose the solids! We continued to track the process parameters, and the plant effluent, trending all of the values. We were in full compliance. We just had a "slight odor" problem, which was, well, able to make the newspaper headlines, Letters to the Editor, and even a cartoon on the Opinion Pages! We operated and waited.

One night, one of our shift supervisors called me at home, and said that I would have to come in to work, as I would not believe what was happening. I arrived shortly thereafter. It was amazing when all of the organisms grew all at once, multiplied like no other event in biological history, and all sloughed off at once! We had zoogleal, sloughed-microbe sludge everywhere! We had trained our personnel well, and they did an excellent job of maintaining the MCRT value during this landslide sloughing..... this one shift filled the empty gravity thickener with biological solids in a matter of 4 to 5 hours! The 140 foot diameter secondary clarifiers held sludge blankets that were within a few feet of their surfaces. The aeration basins were in possession of a very high percentage of MLSS. And then, ONLY THEN, did they call for "help and witnesses"!! No one could have done better, or different. The result would always have been, in a single-word expression, "Impressive!"

As all of you know when a new plant, or plant process is started up, there are just too many process control personnel. Even with all of the engineers, and superintendents on site giving input, no one expected the landslide slough, and therefore, no plan was developed to handle it.


(OK... I am having to lay my plane on her side to get this aerial picture to look like this!)

We then operated our solids dewatering process around the clock to catch up. We were all encouraged as we watched our solids inventory decrease with each passing day. Then, almost three weeks to the day after our "first massive sloughing" we were hit again. Our lab tests and clarifier measurements were compared with the first slugh, and it showed a decrease in solids from the first event. Three weeks later, we were faced with another event. We were on a 3 week slough cycle! Every three weeks that passed, showed further and substantial decreases, until it leveled off.

There is an activated sludge plant down in southern California that shuts down at noon every day, no aeration/ no flow, and automatically restarts every day at 6 pm. The activated sludge is a little tougher than we are sometimes led to believe. With this is mind, if you ever encounter a slough that is overwhelming your ability to thicken it, place it in your digesters, dewater it, etc., you do have some options that you might not normally consider.

1) You may leave the secondary microbes in your clarifiers and aeration basins for a little longer than the textbooks say. The length of time depends on a wide variety of factors such as: the water temperature, flow rates, amount of time the sludge will actually be in the clarifiers, the amount of dissolved oxygen in the MLSS entering the clarifiers, the potential for nitrification/denitrification, etc.

2) For TF/SC plants you can use your sludge reaeration mode to help lessen the impact. I have an attachment that shows the reaeration mode as simply filling one of the aeration tanks with Return Activated Sludge, aerating it, and then mixing this reaerated sludge with the trickling filter effluent

Please refer to the other chapters for information on how to deal with trickling filter problems like odors, filter flies, ponding, etc.

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