给排水专业英语Unit14

Unit14 Activated sludge processThe activated sludge process is now the most widely used biological process for treatment of municipal and industrial wastewaters. Activated sludge process was developed in England in 1914 and was so named because it involved the production of an activated mass of microorganisms capable of aerobic stabilization of the organic content of wastewater.The basic activated sludge process is illustrated by Figure 14.1. The mixture of influent wastewater commonly coming from primary clarifier and returning activated sludge is known as mixed liquor. In activated sludge process, the mixed liquor flows to aeration tank in which the microorganisms responsible for treatment are kept in suspension and aerated. In the aeration tank, contact time is provided for mixing and aerating influent wastewater. Aeration serves at least three important functions (1) mixing the mixed liquor, (2) keeping the activated sludge in suspension, and (3) supplying the oxygen to the biochemical reactions necessary for the stabilization of the wastewater. Once the mixed liquor has received sufficient treatment, excess mixed liquor is discharge into secondary clarifier. The function of the secondary clarifier is to separate the activated sludge solids from the mixed liquor. These solids represent the colloidal and dissolved solids that were originally present in the wastewater. In the aeration unit they were incorporated into the activated sludge floc, which are settleable solids. The separation of these solids, a critical step in the activated sludge process, is accomplished in the secondary or final settling tanks. The excess sludge must be removed before it loses its activity because of the death of the aerobic organisms resulting from lack of oxygen at the bottom of the tank. The most common practice is to waste from the sludge return line. Sometimes waste sludge return line. Sometimes waste sludge is removed by withdrawing mixed liquor directly from the aeration tank. The wasted mixed liquor can then be discharged to a thickening tank or to the primary tanks where the sludge settles and mixes with the raw primary sludge. The waste sludge is further thickened by final sedimentation, centrifuging, or flotation thickening and then treated by biological or chemical means.Since the activated sludge process came into common use , a number of modifications of the conventional activated sludge process have been developed to meet specific treatment objectives:(1) Complete-mix activated sludge process: A completely mixed system can allow a more uniform aeration of the wastewater in the aeration tank. This process has been applied to handle a variety of wastewater with great success, especially because the process can sustain shocks and toxic loads.(2) Step-aeration activated sludge process: In this modified system, influent wastewater is introduced at several point along the aeration tank length. This leads to a relatively homogenous load distribution along the length of the aeration tank resulting in a more efficient use of dissolved oxygen.(3) Contact-stabilization activated sludge: The influent contacts with a high concentration of biomass in a small contact tank of short of time (20 to 40 min). In this contact tank, a rapid biosorption of organic compounds is expected followed by the oxidation of the organic. This system would need smaller tankage and produce smaller amounts of biosolids.(4) Tapered aeration process: In the basic activated sludge process, organic influent is one-point loaded to the head of aeration tank, thus the oxygen demand is extremely high at the head of the aeration tank but very low at the exit end.(5) Higher availability of oxygen for microorganisms leads to improved treatment efficiency and reduced production of biosolids and reactor volume.To ensure high levels of treatment performance with the activated sludge process, several control measure must be considered such as (1) ensuring that there is sufficient level of dissolved oxygen in the aeration tanks, (2) adjusting the amount of return activated sludge ,and (3) controlling the waste sludge. Basically there are several parameters one can refer to as a guideline to find out whether the whole process is operating at peak performance. The parameter used most commonly for controlling the process is solids retention time (SRT, the SRT represent the average period of time during which the sludge has remained in the system). Mixed liquor suspended solids (MLSS) concentration may also be used to determine whether the sludge is healthy or whether it is too old or too young. MLSS concentration can be controlled based on the amount of return activated sludge. The food to microorganism ratio is commonly used to characterize operating conditions. Oxygen uptake rates are also used to monitor and control the activated sludge process.Sludge volume index (SVI) is the measure of the settle ability and compatibility of sludge and is made from a laboratory column settling test (the sludge volume index is defined as `the volume occupied by lg of sludge after it has settled for a specified period of time' generally ranging from 20 min to 1 or 2 hin a 1 or 2 L cylinder. Half an hour is most common settling time)。