水处理专业英语阅读4OperationsofPretreatment.doc

4 Operations of PretreatmentSeveral devices and structures are placed upstream of the primary treatment operation to provide protection to the wastewater treatment plant (WWTP) equipment. These devices and structures are classified as pretreatment because they have little effect in reducing BOD5.Bar RacksTypically, the first device encountered by the wastewater entering the plant is a bar rack. The primary purpose of the rack is to remove trash and large objects that would damage or foul pumps, valves, and other mechanical equipment. Rags, logs, and other objects that find their way into the sewer are removed from the wastewater on the racks. In modern WWTPs, the racks are cleaned mechanically. The solid material is stored in a hopper and removed to a sanitary landfill at regular intervals.Bar racks placed upstream of grit chambers and pump are sized to have an open space of 50 to 150mm between each bar. Racks placed downstream of grit chambers normally have openings of 20 to 50mm, with 25mm being a commonly accepted value. Channel approach velocities are held between 0. 4 and 0. 9m/s. Two channels are provided to allow one to be taken out of service for cleaning and repair.Grit ChambersInert dense material such as sand, broken glass, silt, and pebbles is called grit. If these materials are not removed from the wastewater, they abrade pumps and other mechanical devices, causing undue wear. In addition, they have a tendency to settle in corners and bends, reducing flow capacity and, ultimately, clogging pipes and channels.There are three basic types of grit removal devices: velocity controlled, aerated, and constant level short-term sedimentation basins. We will discuss only the first two, since they are the most common.Velocity controlled. This type of grit chamber, also known as a horizontal flow grit chamber, can be analyzed by means of the classical laws of sedimentation for discrete, non-flocculating particles.Stokes law may be used for the analysis and design of horizontal flow grit chambers if the horizontal liquid velocity is maintained at about 0.3m/s. Liquid velocity control is achieved by placing a specially designed weir at the end of the channel. A minimum of two channels must be employed so that one can he out of service without shutting down the treatment plant. Cleaning may be either by mechanical devices or by hand. Mechanical cleaning is favored for plants having average flows over 0.04m3/s. Theoretical detention times are set at about one minute for average flows. Washing facilities are normally provided to remove organic material from the grit.Aerated grit chambers. The spiral roll of the aerated grit chamber liquid “drives” the grit into a hopper which is located under the air diffuser assembly. The shearing action of the air bubbles is supposed to strip the inert grit of much of the organic material that adheres to its surface.Aerated grit chamber performance is a function of the roll velocity and detention time. The roll velocity is controlled by adjusting the air feed rate. Nominal air flow values are in the range of 0.15 to 0.45 cubic meters per minute of air per meter of tank length m3/ (min m)J . Liquid detention times are usually set to be about three minutes at maximum flow. Length-to-width ratios range from 2：5 to 5：1 with depths on the order of 2 to 5m. Grit accumulation in the chamber varies greatly, depending on whether the sewer system is a combined type or a separate type, and on the efficiency of the chamber. For combined systems, 90m3 of grit per million cubic meters of sewage (m3J105m3) is not uncommon. In separate systems you might expect something less than 30m3/lOtm3. Normally the grit is buried in a sanitary landfill.ComminutorsDevices that are used to macerate wastewater solid (rags, paper, plastic, and other materials) by revolving cutting bars are called comminutors. These devices are placed downstream of the grit chambers to protect the cutting bars from abrasion, They are used as a replacement for the downstream bar rack but must be installed with a hand-cleaned rack in parallel in case they fail.EqualizationFlow equalization is not a treatment process, but s technique that can be used to improve the effectiveness of both secondary and advanced wastewater treatment processes. Wastewater does not flow into a municipal wastewater treatment plant at a constant rate; the flow rate varies from hour to hour, reflecting the living habits of the area served. In most towns. the pattern of daily activities sets the pattern of sewage flow and strength. Above-average sewage flows and strength occur in mid-morning. The constantly changing amount and strength of wastewater to be treated makes efficient process operation difficult. Also, many treatment units must be designed for the maximum flow condition encountered, which actually results in their being oversized for average conditions. The purpose of flow equalization is to dampen these variations so that the wastewater can be treated at a nearly constant flow rate. Flow equalization can significantly improve the performance of an existing plant and increase its useful capacity. In new plants, flow equalization can reduce the size and cost of the treatment units.Flow equalization is usually achieved by constructing large basins that collect and store the wastewater flow and from which the wastewater is pumped to the treatment plant at a constant rate. These basins are normally located near the head end of the treatment works, preferably downstream of pretreatment facilities such as bar screens, comminutors, and grit chambers. Adequate aeration and mixing must be provided to prevent odors and solids deposition. The required volume of an equalization basin is estimated from a mass balance of the flow into the treatment plant with the average flow the plant is designed to treat. The theoretical basis is the same as that used to size reservoirs.With the screening completed and the grit removed, the wastewater still contains light organic suspended solids, some of which can be removed from the sewage by gravity in a sedimentation tank. These tanks can be round or rectangular, are usually about 3.5m deep, and hold the wastewater for periods of two to three hours. The mass of settled solids is called raw sludge. The sludge is removed from the sedimentation tank by mechanical scrapers and pumps. Floating materials, such as grease and oil, rise to the surface of the sedimentation tank, where they are collected by a surface skimming system and removed from the tank for further processing.Primary sedimentation basins (primary tanks) are characterized by Type II flocculant settling. The Stokes equation cannot be used because the flocculating particles are continually changing in size, shape and, when water is entrapped in the floc, specific gravity. There is no adequate mathematical relationship that can be used to describe Type II settling. Laboratory tests with settling columns are used to develop design data.The behavior of the primary tank is analogous to the behavior of clarifiers used in water treatment, with a few important exceptions. The solids concentration is much lower in a primary tank than in a water treatment plant clarifier. Thus, the overflow rates and weir loading rates differ appreciably. Overflow rates are commonly in the range 25 to 60m/d. The Great Lakes-Upper Mississippi River Board of State Sanitary Engineers (GLUMRB) recommends that weir loading (hydraulic flow over the