Secondary treatment is not just a second step of primary treatment. It differs significantly in the type of treatment provided. Whereas primary treatment consists of a separation of solids and liquids through physical processes driven by gravity, secondary treatment consists of a biological processing of the organic waste products contained in the wastestream.

After passing through the primary treatment units where solids are separated from liquids, the liquid portion of the wastestream flows on to the secondary treatment processes. The District’s secondary treatment process is called the “trickling filter/solids contact” process. This process consists of three main treatment components, namely: 1. biotowers, 2. solids contact basins, and 3. secondary clarifiers. The trickling filter component of the process is provided by units called biotowers which are a newer version of traditional trickling filter treatment units.

Biotowers and trickling filters are relatively simple treatment units. Simply stated, they are nothing more than a round tank full of media (rocks or engineered plastic) over which the process flow is distributed, or “trickled.” The District’s two biotowers are 120 feet in diameter with a plastic media depth of 24 feet. The three trickling filters range in size from 145 feet to 210 feet in diameter. The depth of the rock media in the trickling filters is 6 to 8 feet.

The media, whether plastic or rock, is used as a surface on which to grow a biological mass which will contain and support a population of microorganisms which perform the actual treatment that takes place in the biotowers and trickling filters. The media is designed to provide the desired characteristics of ventilation, durability, and surface area. The new plastic media used in the biotowers provides much greater surface area per unit of volume than the rock. It is much lighter and provides better ventilation and water distribution.

The biological growth on the media is where all of the work gets done. The growth is called zoogleal mass or biomass. The biomass forms as a jelly-like mass or slime layer over the surface of the media. The mass consists of microorganisms or “bugs”, primarily bacteria, which feed on the organic waste products contained in the process flow. As the liquid passes over the surface of the biomass, the bacteria feed on and digest these wastes, transforming and breaking them down into more treatable and less polluting forms of matter. If the media is analogous to houses where bacteria live and the water being treated is the food that they eat, then the mechanism that distributes the flow over the media would be considered the food delivery service. The distributor is what most people consider the defining characteristic of trickling filters. The distributor mechanism rotates above the top of the media and sprays the water over the media surface. The distributors at the District have four arms through which the water is sprayed. These arms have openings, or spray nozzles, along their leading and trailing sides. The rotational force for the mechanism is provided by the jet action of the nozzles on the trailing side of the arms. The nozzles on the leading side of the arms provide a counteracting braking action to control the speed of the distributor. The openings are designed to allow varying amounts of flow to spray out along the length of the arms. This is so that there is an equal distribution of water over the entire surface of the media. Because the outside edge of the arm travels at a greater speed and covers more area as compared to the arm near the center as it rotates, an increasingly greater volume of spray must be applied through the openings as the flow travels towards the outside edge of the arm.

After passing through the media, the flow drains through the bottom of the media and drains out of the biotower and trickling filter units. Some of the flow is recirculated back through the biotowers to maintain a constant wetting rate while the remaining flow proceeds on to the solids contact basins.

The solids contact basins are a set of rectangular concrete tanks through which the process flow passes. The primary purpose of the solids contact process is to condition the solids particles discharged from the biotowers to settle readily in the secondary clarifiers. The conditioning of the solids is accomplished by microorganisms that produce polymer, called exocellular polymer, under controlled conditions. The polymer produced causes solids particles to flocculate together into larger clumps of solid particles which settle very effectively in the secondary clarifiers. The conditions under which the microorganisms will properly condition the solids to flocculate are created by supplying the right concentrations of dissolved oxygen in the liquid/solids mixture (called mixed liquor), controlling the solids concentration of the mixed liquor, and by controlling the total time that the solids particles are retained within the solids contact process. Dissolved oxygen is required for the respiration of the microorganisms. The dissolved oxygen level required is about 2.0 parts per million. The oxygen is provided and controlled by an aeration system comprised of blowers and diffusers. Blowers supply air through fine bubble membrane diffusers mounted on the floor of each solids contact basin, similar to the aeration in home fish aquariums. As the air is diffused into the bottom of the basins the mixed liquor is gentlymixed to keep the solids in suspension and oxygen is imparted to the liquid volume. Each basin has a system of aeration diffusers mounted on the floor of the basin.

There are two basic mechanisms to maintain the desired concentration of solids in the mixed liquor; 1. the solids that settle and are removed from the secondary clarifiers are returned by pumping to the head of the solids contact process and 2. a determined amount of solids are removed (wasted) from the process every day. The wasted solids are pumped to the dewatering facility where they are thickened by gravity belt thickeners to approximately 4 to 5 percent solids concentration before being pumped to the primary digesters.

After passing through the solids contact basins flow continues to the secondary clarifiers. The basic purpose of the secondary clarifiers is the same as for the primary clarifiers, to separate solids from liquids through the process of gravity sedimentation. The mixed liquor from the solids contact basins flows into the center of the secondary clarifier tanks through a vertical inlet pipe. The inlet pipe has rectangular openings that allow the flow to disperse from the center of the clarifier tank. A circular baffle or flocculation well is located at the center of the clarifier mechanism. The sides of the flocculation well extend several feet below the surface of the liquid level in the tank. The well serves two main purposes. It directs flow downward in a vertical flow pattern to minimize short circuiting potential of solids particles passing directly from the inlet across the top of the tank liquid level to the weir and out of the clarifier. The well also serves as a stilling basin that promotes flocculation and settling of solids. As flow enters the relatively large volume of the secondary clarifiers its velocity slows considerably. The quiescent characteristic of the flow volume in the clarifier is designed to promote the settling of solids to the bottom of the tank while allowing the clarified liquid to flow over a weir at the perimeter of the tank and on to the chlorine contact basins for disinfection.

Solids at the bottom of the clarifier are withdrawn by a rotating sludge collection mechanism. The rotating mechanism differs from the primary clarifier sludge rake system that is designed to physically rake or push sludge to a centrally located hopper from which it is withdrawn by pumping. The secondary clarifier mechanism removes sludge by hydraulic suction. The sludge collection arms of the mechanism are constructed of hollow tubes with inlet orifices on the leading side of the tube. The tubes are connected to the suction side of the return sludge pumps through a center ring and seal assembly at the center bottom of the clarifier. As the pumps draw suction flow through the collection arms of the clarifier mechanism, settled sludge is drawn into the tubular arms through the inlet orifices and pumped back to the inlet side of the solids contact basins.

The primary reason for using the hydraulic suction type clarifier mechanism for the District’s application is to accommodate rapid sludge removal. Rapid sludge removal is very important in the trickling filter/solids contact process. The process is designed to provide conditions which promote the biological production of polymers which in turn promote flocculation and settling. Formation and preservation of the biopolymer is sensitive to the concentration of dissolved oxygen in the process flow. If the dissolved oxygen level is too low biopolymer production decreases and becomes ineffective. The longer settled solids remain in the clarifier, the greater the chance of creating low dissolved oxygen conditions which cause low biopolymer production and poor solids settling. That is why rapid sludge removal is important to the trickling filter/solids contact process.

As previously explained, to operate the trickling filter/solids contact process effectively the concentration of solids, or mixed liquor, in the solids contact basins must be controlled and maintained at selected levels. The concentration and age of mixed liquor correlates to the number and type of microorganisms in the mixture. Two methods of control are used to maintain the mixed liquor concentration at the desired level; 1. recirculating secondary clarifier sludge back to the solids contact basin and 2. wasting secondary sludge out of the process. Wasted sludge is sent to the digesters for treatment where it mixes with raw sludge from the primary clarifiers. Secondary sludge is much less concentrated than raw sludge, about 0.25 to 0.5 percent compared to 3.5 to 5.0 percent for raw sludge. To avoid dilution of the digester contents with the less concentrated solids from the solids contact process, wasted solids are thickened prior to being pumped to the digesters. The thickening process is accomplished in the dewatering facilty using gravity belt thickening equipment. Gravity belt thickeners operate in the same manner as the first section of belt filter presses. As the waste secondary sludge is fed to the gravity belt thickeners polymer is added as a dewatering aid. The polymer causes the solids particles to flocculate into larger conglomerate particles. Flocculation greatly enhances the ability to separate the liquid and solids. As the waste sludge is fed to inlet of the gravity belt thickeners it is distributed evenly across a plate of the inlet and flows onto a moving porous mesh belt. Because the belt is porous and the solids have been flocculated by the polymer, water drains readily through the belt while the solids are retained on top of the belt. V-shaped plows are positioned to ride on the top surface of the belt as it conveys the solids. The function of the plows is to move the solids from side to side as they move with the belt and to create exposed furrows of belt for the water to drain through. The speed of the moving belt and specific dose of polymer are adjusted to achieve the desired thickness of sludge produced by the gravity belt thickener. As solids are discharged from the gravity belt thickener they are pumped to the digesters for treatment.