TRICKLING FILTER DESIGN:

Walls are usually constructed of concrete, while some small installations may be made of steel. Some others have no walls, or very short small walls, as the media is placed directly on concrete underdrain system. Colder climates warrant walls of concrete and usually covers to retain warmth, to reduce freezing tendencies.

The distribution system consists of arms attached to a central mechanism, which is usually powered by the hydraulic force of the water being spread over the surface of the media. The biofilters that we installed in my last job, required a head of approximately 1.5 to 2.0 feet to create the pressure necessary for the arms to rotate. There are weirs placed in the center of the column to allow low flow rates to go out two arms. As the flow rate increases, the water rises in the center box, and then over flows the higher weirs, allowing water to now go down the other two arms. The distribution system should be checked for even flow distribution by placing containers at intervals from the center to the outer reaches of the media, and rotating the arms over them one or two revolutions, then checking for equal amounts of water in each container. During changes in temperature, especially in very cold climates, the arms are adjusted for proper level, due to the contraction/expansion of the cables suspending the arms and the arms themselves.

The media of a new installation is usually a fiberglass or plastic-type modular product. It has the advantage of a greatly increased surface area per unit of volume, over slag and rock medias. "Random Piece Media", such as rings, cylinders, or in other types provide surface areas ranging from 29 to 70 sq ft per cu ft. of volume. The plastic/fiberglass module types, which are "cubes" of media, have a surface area of 12 to 35 sq ft per cubic foot of volume. I have found that one needs to place fiberglass grating from the entrance to the filter out to the center column, especially if plastic/fiberglass media is utilized, to help prevent the destruction of the top layer due to foot traffic.

The underdrain is usually a false floor using utilizing poured concrete, vitrified clay blocks or other inert materials. It is important to provide sufficient ventilation and a path for the filter effluent to pass through simultaneously. The underdrain should be designed to provide at least 15 to 20 % ventilation area. This is especially critical in the construction of a high rate filter. Fans, providing forced air ventilation, may be installed to provide a more positive and reliable oxygen supply to the media and the growths on it. If fans are installed, downward flow of air is more desirable, as the odors generated are usually far less (primary effluent odors are not blown immediately out of the top, and also the downward flow of air tends to reduce the odors by growing organisms that will help purify the water and odors that are generated).

ROTATING BIOLOGICAL CONTACTORS (RBCs) DESIGN:

The structural media is fixed a horizontal shaft which rotates in a tank or a fixed channel. The discs are constructed of a plastic or fiberglass material, which is "corrugated" like corrugated panels for roofing/greenhouse construction, or otherwise molded to increase surface area as in trickling filter modules.

The units rotate at about two to five rpm, exposing the media to the water and the atmosphere alternatively to cause oxygen transfer and then to pick up "food" for the microbes to stabilize. Design literature states that the media is passed through the waste stream about 40 per-cent of the time, with the remaining 60 percent of the time exposed to the atmosphere to ensure sufficient oxygen transfer. They are usually installed in a series of reactors, allowing for the first stages to have the heaviest loadings, with reduced loadings and possible nitrification in the latter stages. The zoogleal film sloughs off the media as in the biofilter types, to be settled in a secondary sedimentation. There are very few installations that practice recirculation with RBCs.

TRICKLING FILTER OPERATION & MAINTENANCE:
The upper reaches of the media supports a fast growing microbial population, while the bottom levels of the media is almost starved for nutrients. The lower levels, in lightly loaded filters are usually able to support nitrification (conversion of ammonia into nitrates, a future article).

It is essential to keep the orifices in the arms open and free from debris. The media should be cleaned of all debris at least weekly also. Insure that the oil level, and proper oil changes are made on the distributor bearing. The mercury seals to assist in keeping the water out of the bearing are a thing of the past! The "modern" seals do fail, and do so with a greater frequency than desired!

Psychoda the wonderful filter fly, is a very small, "fuzzy" insect that loves biofilters! It feels that its sole purpose in life is to enter operator’s respiratory and digestive tracts, causing sneezing, gagging, and other great reactions! It breeds in trickling filters, and is difficult to totally eliminate. Proper wetting, meaning no accumulations of debris on the media, landscaping of bare necessity near the biofilters, etc. will help control their numbers. Other tactics include: flooding the media to drown the larvae (not a good idea if your biofilters are NOT CONSTRUCTED to do this!!); chlorine dosing; ap-plying insecticides; increasing the filtration rate for a short period of time to wash the larvae off the media; biofilters with covers seem to have fewer problems with filter flies. The biofilters that have the higher hydraulic loading rates appear to have fewer Psychoda problems also.

Ponding occurs when water pools on the surface of the media. This is an indication that the media is clogged below it. Several methods have provided some temporary relief: Increasing the rate of recirculation for a few hours to hydraulically "slough" the media of the excessive growth; Removing a filter from service for a few days (which dries out the excessive growth); turning off the distributor arm rotation, and using a fire hose on the affected area to wash the surface free of the obstruction; and applying a chlorine dosage up to 2 to 3 mg/L to the filter during low flow periods for a few hours. DO NOT dose at any rate higher, as it will cause a removal of the zoogleal film and a poor quality of effluent will result!

Odors may be reduced by increasing the recirculation rate; adjusting the primary sedimentation process sludge pumping rates and wastewater stream detention time if applicable to produce a fresher effluent; and increasing forced air ventilation.

ROTATING BIOLOGICAL CONTACTORS (RBCs) OPERATION & MAINTENANCE:

RBCs have a history of the media becoming plugged with the biological slime growth (zoogleal film), and therefore unable to properly treat the wastewater. Washing the media with fire hoses while the unit is rotating helps to keep the media open. Debris passing from the primary sedimentation tanks has been known to accumulate in the bottom of the tank, and stopping the rotation of the media. Usually this is at facilities that have poor primary sedimentation or do not have sedimentation preceding this process. The media has also been known to fail, by breaking off of the shaft upon which it rotates. Replacement of the media is the obvious cure to this problem.

Snails seem to be the biggest headache of operators where they have established themselves. These gastropods can accumulate to great quantities, and have been known to even stop the rotation of the media in the tank, by their settling in the bottom of the tank!

RBCs designed for nitrification will be very lightly loaded as compared to those not designed for this type of effluent parameter. The hydraulic loading rates are also much lower, to enhance the creation of higher dissolved oxygen levels required for successful nitrification.

Insure that the oil level, and proper oil changes are completed on the drives, chains, air compressors, etc. Proper maintenance of the protective coatings on the metal surfaces of the bearings, gear boxes, motors, etc should be completed.

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