ENVIRONMENTAL ENGINEERING. Chemical Engineering department

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1 ENVIRONMENTAL ENGINEERING Chemical Engineering department

1- PRELIMINARY AND PRIMARY TREATMENT Screening is the first technique employed in primary treatment, which is the first step in the wastewater treatment process.

The screening process is used primarily to present the clogging and interference of the following wastewater treatment processes.

2 1- PRELIMINARY AND PRIMARY TREATMENT Screening is the first technique employed in primary treatment, which is the first step in the wastewater treatment process. This step removes all sorts of refuse that has arrived with the wastewater such as plastic, branches, rags, and metals. The screening process is used primarily to present the clogging and interference of the following wastewater treatment processes. Screens are considered coarse if their openings are larger than 6mm, fine if their openings are between 1.5 and 6mm, and very fine if their openings are between 0.2 and 1.5mm. A typical bar screen. Screens are cleaned manually if the object caught is larger and mechanically if finer particles are caught. The angle of the screen may also be varied to affect the efficiency of filtration. In order to remove coarse solids, numerous types of detritus tanks, grinders, and cyclonic inertial separation are utilized, including a comminutor and a grit chamber. The type of grit removal separation depends upon the characteristics of the grit itself. A typical comminutor. A grit chamber used in wastewater treatment.

2- Primary settling tanks (or basins) These are usually large tanks in which solids settle out of water by gravity (figure below) where the settle-able solids are pumped away (as sludge), while oils

The usual detention time is 11/2 21/2 hours. Longer periods usually result in depletion of dissolved oxygen and subsequent anaerobic conditions.

3 2- Primary settling tanks (or basins) These are usually large tanks in which solids settle out of water by gravity (figure below) where the settle-able solids are pumped away (as sludge), while oils float to the top and are skimmed off. It operates by means of the velocity of flow is reduced to about m so that the suspended material (organic settleable solids) will settle out. The usual detention time is 11/2 21/2 hours. Longer periods usually result in depletion of dissolved oxygen and subsequent anaerobic conditions. Removal of suspended solids ranges from per cent, and a % reduction of the five-day biochemical oxygen demand (BOD) refer the amount of oxygen required by the microbes within the wastewater to digest the matter that they are using for food. By removing these solids early on, the efficiency of the microbial digestion at later stages in increased Primary settling tank schematic Primary settling tank

4 3- Secondary Treatment The water leaving the primary clarifier has lost much of the suspended organic matter but still contains a high demand for oxygen due to the dissolved biodegradable organics. This demand for oxygen must be reduced (energy expended) if the discharge is to avoid creating unacceptable conditions in the watercourse. The objective of secondary treatment is to remove BOD while, by contrast, the objective of primary treatment is to remove solids. Except in rare circumstances, almost all secondary treatment methods use microbial action to reduce the energy level (BOD) of the waste (a process advocated in the late 1800sby Dibdin and Dupré, Fixed Film Reactors Although there are many ways the microorganisms can be put to work, the first really successful modern method of secondary treatment was the trickling filter. The trickling filter, shown in Figure below, consists of a bed of media (such as fistsized rocks or various plastic shapes) over which the waste is trickled. An active biological growth forms on the media, and the organisms obtain their food from the waste stream dripping over the bed. Air is either forced through the media or, more commonly, air circulation is obtained automatically by a temperature difference between the air in the bed and ambient temperature. In older filters the waste is sprayed onto the rocks from fixed nozzles; newer designs use a rotating arm that moves under its own power, distributing the waste evenly over the entire bed, like a lawn sprinkler. Often the flow is recirculated, obtaining a higher degree of treatment. The name trickling filter is obviously a misnomer because no filtration takes place. A modern modification of the trickling filter is the rotating biological contactor, or rotating disc, the microbial growth occurs on rotating discs that are slowly dipped into the wastewater, which provides their food. By bringing the discs out into the

5 open air, the microbes are able to obtain the necessary oxygen to keep the growth aerobic. Suspended Growth Reactors Around 1900, when trickling filtration was already firmly established, some researchers began musing about the wasted space in a filter taken up by the rocks. Could the microorganisms be allowed to float free, and could they be fed oxygen by bubbling in air? Although this concept was quite attractive, it was not until 1914 that the first workable pilot plant was A trickling filter. Constructed. It took some time before this process became established as what we now call the activated sludge system. The key to the activated sludge system is the reuse of microorganisms. consists of a tank full of waste liquid (from the primary clarifier) and a mass of microorganisms. Air is bubbled into this tank (called the aeration tank) to provide the necessary oxygen for the survival of the aerobic organisms. The microorganisms come into contact with the dissolved organics and rapidly adsorb these organics on their surface. In time, the microorganisms use the energy and carbon by decomposing this material to CO2, H2O, and some stable compounds and in the process produce more microorganisms. The production of new organisms is relatively slow, and most of the aeration tank volume is used for

this purpose. Once most of the food has been used, the microorganisms are separated from the liquid in a settling tank, called a secondary or final clarifier. The liquid escapes over a V-notch weir.

These microorganisms are said to be activated hence, the term activated sludge. Rotating disc fixed-film biological schematic diagram of the activated sludge system.

6 this purpose. Once most of the food has been used, the microorganisms are separated from the liquid in a settling tank, called a secondary or final clarifier. The liquid escapes over a V-notch weir. The separated microorganisms exist on the bottom of the final clarifier without additional food and become hungry waiting for more dissolved organic matter. These microorganisms are said to be activated hence, the term activated sludge. Rotating disc fixed-film biological schematic diagram of the activated sludge system. When these settled and hungry microorganisms are pumped to the head of the aeration tank, they find more food (organics in the effluent from the primary clarifier), and the process starts all over again. The sludge pumped from the bottom of the final clarifier to the aeration tank is known as return activated sludge. The activated sludge process is a continuous operation, with continuous sludge pumping and clean water discharge. Unfortunately, one of the end products of this process is excess microorganisms. If the microorganisms are not removed, their concentration eventually increases to the point where the system is clogged with

7 solids. It is, therefore, necessary to waste some of the microorganisms, and this waste activated sludge must be processed and disposed of. Its disposal is one of the most difficult aspects of wastewater treatment. Activated sludge systems are designed on the basis of loading, or the amount of organic matter (food) added relative to the microorganisms available. This ratio is known as the food-to-microorganisms ratio (F/M) and is a major design parameter. Unfortunately, it is difficult to measure either F or M accurately, and engineers have approximated these by BOD and the suspended solids in the aeration tank, respectively. The combination of the liquid and microorganisms undergoing aeration is known (for some unknown reason) as mixed liquor, and the suspended solids are called mixed liquor suspended solids (MLSS). The ratio of incoming BOD to MLSS, the F/M ratio, is also known as the loading on the system and is calculated as pounds of BOD/day per pound of MLSS in the aeration tank. If this ratio is low (little food for lots of microorganisms) and the aeration period (detention time in the aeration tank) is long, the microorganisms make maximum use of available food, resulting in a high degree of treatment. Such systems are known as extended aeration and are widely used for isolated sources (e.g., motels and small developments). Added advantages of extended aeration are that the ecology within the aeration tank is quite diverse and little excess biomass is created, resulting in little or no waste activated sludge to be disposed of a significant saving in operating costs and headaches. At the other extreme is the high-rate system, in which the aeration periods are very short (thus saving money by building smaller tanks) and the treatment efficiency is lower.

Sanitary and Environmental Engineering I (4 th Year Civil)

Sanitary and Environmental Engineering I (4 th Year Civil) Prepared by Dr.Khaled Zaher Assistant Professor, Public Works Engineering Department, Faculty of Engineering, Cairo University Wastewater Flow