Wastewater Treatment clarifier

Transcription

1 Wastewater Treatment Pretreatment During pretreatment, items would normally be removed that would hinder the further processes of treatment. Items commonly removed include roots, rags, cans, or other large materials. Primary Treatment The area where the solids that readily settle out are allowed to do so, and are then removed in a clarifier. The clarifier collects the solids on the bottom and places them into a sludge basin. It also collects floatables, called scum and removes it as well. These are pumped to the sludge handling facilities. Secondary Treatment Secondary treatment uses living organisms to remove the nonsettleable organic material. The organisms, such as bacteria, feed on the organic material and release byproducts, which are easily removed. There are primarily two types of secondary treatment, trickling filters and activated sludge. Trickling Filters A trickling filter is a bed of somewhat small rocks upon which grows a film containing the biological media. Wastewater is then trickled over the media and allowed to filter through the rock bed. The organisms on the rock feed on the organic material in the water. This is an aerobic treatment. The effluent has dead biological matter, which has sloughed off, called humus. This effluent then goes to a secondary clarifier. Activated Sludge Activated sludge another type of aerobic environment in which a media is suspended through a tank which contains organisms. The media has air bubbled through it so there is a supply of oxygen for the organisms. The effluent is called mixed liquor and contains a large number of biological organisms, both dead and alive. The mixed liquor is pumped to the secondary clarifier for treatment. Secondary Clarifier The secondary clarifiers do much the same thing as the primary clarifiers, except the waste that settles to the bottom is either humus from the trickling

2 filter or mixed liquor from the activated sludge tank. Either way it is composed of old organisms, which are pumped to the sludge handling facilities. Tertiary Treatment Tertiary treatment is used to remove specific contaminants. The facilities in Bloomington use sand filters to remove fine inorganic suspended material. Also, if needed, the effluent could be chlorinated to remove pathogenic organisms. According to current Environmental Protection Agency (EPA) rules, there cannot be residual chlorine in an effluent. Therefore a chlorinated effluent must be treated with sulfur dioxide (SO 2 ) to remove any chlorine residual. Another way to disinfect an effluent is to use ultraviolet (UV) radiation. This is a good way to avoid using dangerous chemicals, but the cost is considerably higher. Dissolved Oxygen (DO) Aerobic conditions need oxygen for the biological reactions. The oxygen described is dissolved in solution, with its concentration measured in mg/l. The dissolved oxygen (DO) is carefully monitored because it is a good indicator of treatment processes. Aerobic Digestion Aerobic Digestion is the breakdown of organic matter by organisms that utilize oxygen. Final components are H 2 O and CO 2. Anaerobic Digestion Anaerobic digestion is the breakdown of organic matter by organisms without the presence of oxygen. This is a two step process where the complex organic material is broken down by acid formers to organic acids. Then methane fermenters further breakdown the organic acids into methane (CH 4 ), carbon dioxide (CO 2 ) and other products, including H 2 O and hydrogen sulfide (H 2 S). Nitrogen Cycle One of the most important duties of wastewater treatment is the removal of dissolved ammonia. This is accomplished once again by bacteria. A bacteria called Nitrosomonas converts the ammonia nitrogen (NH 3 -N) into the nitrate nitrogen (NO 2 -N) form. Then a bacteria called Nitrobacter converts the NO 2 -N into the nitrate nitrogen form (NO 3 -N) form. These

3 forms can them be reduced by denitrification back to atmospheric nitrogen (N 2 ). Sludge Sludge or biosolids is a byproduct of the treatment process. It is placed in an anaerobic environment and allowed to digest. The resulting slurry must be dewatered before disposal. It can be placed in sand drying beds and left to drain and dry or it can be acted upon mechanically. These include using a centrifuge to spin the water out or using a sludge press. In either case a hyphobic polymer is added to aid in dewatering. The sludge can then be taken to disposal. This can be done by removal to a landfill or by incineration and then removal to a landfill. It can also be used in land application if the contaminants are not too high.

4 Thought Questions 1. If you found a large drop in the activated sludge, what could you conclude happened? 2. If you found a large increase in DO in the activated sludge, what could have happened?

5 Water Reclamation Field Trip 1. What is BOD? 2. Identify the functions of the following: a. screens b. grit chambers c. aerobic digestors d. anarobic digestors e. filters 3. Which digestor above operates in the absence of oxygen? 4. What new chemical was just added to the system to help cut down on the odor emitted from the plant? 5. How many different types of fish are supported in Sugar Creek?

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