Texas Waste Water Treatment Ch#09 Fixed Film Systems

Wastewater applied to a trickling filter usually has had the large solids removed in a primary clarifier. Only the finely divided, suspended solids, the colloidal solids, and the dissolved solids remain.

Favorable environmental conditions for maximum biological activity must be maintained in the trickling filter at all times. These conditions include: continuous food supply; moist zoogleal mass; and aerobic conditions.

There are five basic elements (parts) in a trickling filter: Filter floor, Underdrain system, Walls, Filter media, and Distribution arms. Additional elements are the inlet, outlet, ventilation ports, distributor base and bearings, orifices, and center well.

The main purpose of the media is to provide a surface on which the microorganisms can grow. Media must be durable because treatment plants are designed for many years of service. Various media materials have been used, including stone, slag, ceramics, coal, and redwood blocks, or slats. Synthetic materials, such as molded plastic, also have come into use. They are lightweight and easy to handle, allowing greater filter depth without excess weight on underdrains and floor. They double the surface area exposed for the growth of film and their voids clog less often than those of stone filters.

The answer matches the given review question answer key and is what you are likely to see on an exam and so will remain unchanged but the book has this to say on the topic: "The top layer of a properly operating filter bed is a green algae growth which extends only 2–6 inches down or as far as sunlight will penetrate. Algae growth is dependent on sunlight, warm weather, and nutrients. Algae growth is not necessary for a trickling filter operation; however, a lack of growth may be an indication the presence of toxic materials, lack of sunlight, or cold weather. Deeper in the bed lies the zoogleal mass, a thick layer of grayish, jelly-like slime where many organisms, such as snails, fly larvae, worms, bacteria, fungi, protozoa, and ciliates, live."

The film (referring to the zoogloeal mass forming on the face of the media) takes the organic matter from the water and gives up its waste products, which include carbon dioxide, water, nitrates, and sulfates. The colloidal or dissolved organic matter is biologically oxidized and converted into stable end products.

When the aerobic portion of the film thickens so that oxygen cannot penetrate the layers, it becomes septic or anaerobic. At this point, portions of the film will slough off (sloughings) and wash out of the filter to a final settling basin. The material that sloughs off the filter, called humus sludge, settles in the clarifier.

The answer matches that given in the review question answer key and is the answer you are likely to see on an exam but the book has this to say on the subject of hydraulic loading units: "The units most commonly used to measure the rate of hydraulic loading are million gallons per acre per day (MGAD) and gallons per day per square foot (gpd/ft.2)"

The units most commonly used to measure organic loading are pounds of BOD/acre ft./day (pafd) and pounds of BOD/1,000 ft.3 /day.

Classification is determined by the hydraulic and organic loading applied to the filter. Filters are classified as follows: Standard-rate, Intermediate-rate, High-rate (rock or manufactured media), Roughing

The filter floor supports the underdrain system, which in turn supports the filter media. The underdrain system carries away the filter effluent and also conveys air to the filter. Underdrains for standard-rate filters are usually designed to flow half-full and for high-rate filters, one-third full.

The amount of organic matter removed by the trickling filter depends on recirculation. Recirculation can be used to reduce the strength of wastewater applied to the filter. Increased hydraulic loading resulting from recirculation will help slough organic matter. This will prevent excess biological mass, which could result in ponding. Recirculation also decreases fly breeding, reduces detention time in the primary clarifier during low flow, reduces odor and septicity in primary clarifiers, and keeps the media wet during periods of low flow in smaller plants.

So the book has a little more than two sentances directly relating to this question. "A graph relating hydraulic filter loading and efficiency shows that efficiency can be expected to drop as the hydraulic loading on a trickling filter increases (Figure 9.12 in the book if you're interested). The efficiency of a trickling filter is measured in percent BOD removal." since this seems more helpful in passing information than understanding I would offer this instead: In a wastewater treatment process unit, hydraulic loading is known as the volume of wastewater applied to the surface of the processing unit per time period. An increase in hydraulic loading means that in a set period of time an increased volume of water is applied to a set or constant amount of surface area. this means that each unit of volume is exposed to that process surface for less time. since it is exposed to the process surface for less time it is not as effected BY that surface as much. in this context that means that the zoogleal mass is less able to catch and hold onto the matter that constitutes BOD and so more passes through the filter with the water. since less BOD is removed over the shortened exposure time the difference between the amount of BOD entering and leaving the process unit approach the same value meaning your process unit is accomplishing less useful work and therefore your efficiency drops. Given this the best test for determining the efficiency of your process unit is to determine how much BOD is being removed by the process. accordingly the BOD present in the influent and effluent can be compared and conveniently represented as an easily understood percentage.

Some common problems encountered in the operation of a trickling filter are ponding or pooling (reffers to an obstruction in the flow path that causes water to collect and interrupts air flow potentially causing anaerobic conditions), filter flies (AKA psychoda are small irritating flies that for some reason love to fly up your nose or into your mouth... honestly much like mosquitoes the less experience you have with them the happier you are likely to be in general), odors (The trickling filter is an aerobic process. Excessive odors indicate an anaerobic condition in the filter bed.), clogging of distribution arms (If orifices are clogged, green algae growth will be lost and unsightly rings will appear around the bed surface. If a large number of orifices are obstructed, excess pressure can rupture the mechanical seal.), snails (A few snails will not affect trickling filter operation. If they increase, however, their shells can damage distributor arms and pumps and fill up digesters.). FUN FACT: You can make use of the unoccupied shells of snails removed from waste treatment processes to make lime by firing them in a camp fire, slaking the calcined shells (calcium oxide CaO) with water so that they disintegrate (into calcium hydroxide Ca(OH)2) in an exothermic reaction (heat producing), and then mixing them into lime putty that's water resistant after it dries. it makes mortar, has several uses in gardening, and is the same lime used in "limelight" famous for its use in theaters where it was heated to produce intense and sustained white light. https://en.wikipedia.org/wiki/Limelight

Ponding will occur when more wastewater is applied than can pass through the filter bed, media is too small or not uniform in size, clogging fills in voids surrounding poor-quality media, high amounts suspended solids reach the filter from poorly operated primary treatment units, excessive sloughing of biomass clogs the voids, durring excessive organic loading, and when growth of biomass in the upper part of the filter becomes too dense.

Filter flies are difficult to control and almost impossible to eliminate. To control them you can flood the filter for a period of several hours at weekly intervals (This prevents the completion of the life cycle.), chlorinate the filter influent once a week to maintain a 1.0–1.5 mg/L residual for several hours (To interrupt the life cycle), keep the inside of the exposed filter wall wet (A stream of water from the end of the distributor arm can be directed to strike the wall above the media), and/or recirculate to keep a continuous flow of liquid on the bed.

Prevention of odors may be accomplished by one or more of the following procedures: Maintaining aerobic conditions in the entire collection system and in the settling tanks, increasing the recirculation rate to minimize the accumulation of excessive growth in the filter bed, chlorinating the filter influent daily for short periods (preferably at night when flow is low), and practicing good housekeeping in all parts of the treatment plant.

Problems with clogged orifices on the distributor arms may be corrected by the following methods: Cleaning orifices and flushing distributor arms when necessary, improving grease and suspended solids removal in the primary clarifier, and lubricating the distributor according to manufacturer's instructions.

Each shaft, stage, or unit is enclosed, usually by fiberglass. Alternatively, the entire plant may be housed in a building. The enclosure prevents excessive heat loss from wastewater. It also eliminates sunlight and the resulting algae growth on the surface of the unit.

The discs are mounted vertically on a central horizontal shaft. The shafts are usually 25 ft. long and the discs are typically 12 ft. in diameter. The bottom of the enclosure contains a basin where the wastewater flows through the length of the system. The discs are mounted with 40% of their surface within the wastewater and 60% above. The discs are slowly rotated at a rate of 1.0 to 1.5 rpm or about 1 ft./sec. peripheral speed. Each unit is driven either by a motor with a gear reducer or by air trapped in air cups. When driven by a motor, the drive assembly on each drum or disc is powered by an electric motor operating at 1,200 rpm. When driven by air, air headers below the disc allow air to fill air cups installed on the periphery of the disc. The air buoyancy turns the disc.

Normally, the basins will function with the discs in continuous operation. Wastewater from the primary clarifier enters the head of the RBC through two influent pipes. The influent wastewater displaces water and sloughed solids into the effluent channel leaving the basin. The displaced water then flows through transfer pipes to the final clarifier for further treatment. The discs are slowly rotated at a rate of 1.0 to 1.5 rpm or about 1 ft./sec. peripheral speed. Each unit is driven either by a motor with a gear reducer or by air trapped in air cups. When driven by a motor, the drive assembly on each drum or disc is powered by an electric motor operating at 1,200 rpm. When driven by air, air headers below the disc allow air to fill air cups installed on the periphery of the disc. The air buoyancy turns the disc.

Shortly after startup, organisms naturally present in wastewater begin to cling to the rotating surface of the disc, where they multiply. In a week or two, the entire wetted surface area of the disc becomes covered by a 0.05–0.1 in. film of gray to brown biological growth. The attached biomass, containing approximately 50,000 mg/L solids, has a shaggy, stringy appearance and consists of a large population of biologically active organisms that remove the organic materials from the wastewater.

RBCs receive their influent from a primary clarifier or a fine screen. The effluent from the RBC unit flows to the final clarifier, where the excess biomass or humus settles from the wastewater.

Common problems in operating RBCs include the following: • Toxic materials in the wastewater and high or low pH values will upset the process but only for a short time. Because of the shaggy filamentous growth of the biomass, only the more exposed organisms will be affected. Operation will return to normal in a few days. • Extended low flow may cause excessive sloughing of the biomass. This condition will clear up when normal flow returns. • A white biomass over the entire disc is probably caused by high influent hydrogen sulfide concentrations. It may also be an early indication of organic overload or low DO. The sulfur bacteria Beggiatoa are typically responsible for the white color. They use sulfur compounds as an energy source. If Beggiatoa develop in combination with nitrifier bacteria, the smooth slime will look pink (a mix of the red and white organisms). • A buildup of solids in the tank of the contactor is probably caused by poor primary treatment. It must be determined whether the solids are organic or inorganic. The solids should be removed and grit removal or primary clarifier efficiency improved. • Decreased efficiency of an RBC unit may be due to low wastewater temperature, unusual variations in hydraulic or organic loading, or a high or low pH value. The appearance of the biomass may indicate operational problems. A black biomass could be an indication of an organic overload or low dissolved oxygen content in the tank effluent.

A buildup of solids in the tank of the contactor is probably caused by poor primary treatment. It must be determined whether the solids are organic or inorganic. The solids should be removed and grit removal or primary clarifier efficiency improved.