控制氧化沟工艺中的污泥沉降外文翻译三.doc

本科生毕业设计外文文献翻译Controlling sludge settleability in the oxidation ditch processK.J. HartleyABSTRACTThis paper describes an investigation aimed at developing an operating technique for controlling sludge settleability in the oxidation ditch form of the nitrification denitrification activated sludge process. It was hypothesized that specific sludge volume index (SSVI) is lowest at an optimum process anoxic fraction and increases at higher and lower fractions. Using effluent ammonia:nitrate ratio as a surrogate for anoxic fraction, it was found that a simple empirical model based on a three solids retention time moving average nitrogen ratio was able to replicate the long-term SSVI variations in two independent oxidation ditches at a full-scale plant. Operating data from a second oxidation ditch plant during periods when a prefermenter was on- or off-line showed that SSVI also varies with RBCOD, greater RBCOD giving lower SSVI. It was concluded that best settleability occurs at about the same anoxic fraction as lowest effluent total nitrogen concentration, with an ammonia:nitrate ratio of about 1. An operating rule of thumb is to use dissolved oxygen control to maintain effluent ammonia and nitrate nitrogen concentrations about equal. A third oxidation ditch plant deliberately operated in this manner achieved 15-month median operating values for SSVI of 60 mL/g and for effluent ammonia, nitrate and total N, respectively, of 0.2, 0.3 and 2.0mgN/L. KeywordsSettleability ；Sludge volume index；SVI；Control；Nitrogen；Oxidation ditch1 IntroductionThe oxidation ditch process has the capability to reliably meet low total nitrogen standards because of its special operating characteristics. Nitrogen removal can be continuously optimized in operation by varying the anoxic and aerobic mass fractions to meet short- and long-term changes in wastewater characteristics (Hartley, 1997; Mines and Woods, 1994). It has also been observed that sludge settleability in oxidation ditches varies with the operating mass fractions (Vivian and Hartley, 1989).Sludge settleability is one of the most important design and operating characteristics of the activated sludge process. It affects both the sizing of tankage and the process capacity actually achieved. Yet, nearly 100 years after the activated sludge process was invented (Ardern and Lockett, 1914), positive control of this parameter remains elusive. Four slightly different tests are in use for measurement of settleability, sludge volume index (SVI, employing simple cylinder settling, Mohlman, 1934), stirred SVI (sSVI, in which the cylinder is fitted with a stirrer, APHA et al, 1998), diluted SVI (DSVI, using serial dilutions of the sludge, Koopman and Cadee, 1983) and SSVI3.5, or simply SSVI (stirred specific volume index, in which the sludge concentration is standardized at 3.5 g/L, White, 1975). The values of these parameters can be related to the settling properties of the sludge, and to each other, through published statistical correlations (Daigger, 1995; Ekama et al., 1997; Ozinsky and Ekama, 1995).A range of factors affects settleability in activated sludge processes, including feed quality, solids retention time (SRT), mixing characteristics, oxygen regime, pH and nutrient availability (Jenkins et al., 2004). For N and N&P removal processes, the Water Research Group at the University of Cape Town has conducted a comprehensive settleability study, which they published in a lengthy series of papers and summarised in Ekama et al. (1996), Casey et al. (1999) and Tsai et al. (2003). Their study included intermittent aeration formats mimicking the oxidation ditch process. They found that the low F/M group of filaments (such as Microthrix parvicella and 0092) cause practically all bulking problems in long SRT nutrient removal processes and that they cannot be controlled by anaerobic, anoxic or aerobic selectors. Whether or not the process includes biological phosphorus removal, it is the operating nitrogen regime that governs sludge settleability. According to the model they developed, in a two-stage anoxicaerobic system utilising mainly slowly biodegradable COD, if denitrification in the primary anoxic zone is incomplete and nitrite is present, the intracellular intermediate nitric oxide persists and inhibits the denitrifying floc-formers in the aerobic zone. The filamentous organisms, which do not compete well in the anoxic zone because they can only denitrify nitrate to nitrite, are not inhibited in the aerobic zone where they proliferate. Intracellular nitric oxide does not accumulate when the COD is readily biodegradable (RBCOD) so a significant fraction of RBCOD relieves the competitive pressure on the floc-formers.Tsai et al. (2003) observed that M. parvicella growth could be stimulated or inhibited by varying the concentration of ammonia (the organisms required nitrogen source) to high or low values, respectively. They therefore postulated that deterioration of settleability at low aerobic fraction can be due to increasing ammonia concentration (decreasing nitrification efficiency).2 The oxidation ditchFig. 1 illustrates the general oxidation ditch reactor format. Mixed liquor is impelled around a circuit by one or more mechanical aerators, or by mixers in the case of diffused air aeration. This provides a typical mixed liquor: feed flow ratio of 100200. The dissolved oxygen (DO) concentration jumps up at the aerator and declines downstream. The DO concentration at the aerator equals the oxygen transfer rate divided by the circulating flow rate. The anoxic fraction is controlled by varying the aeration rate to maintain a DO set point at a probe located towards the downstream end of the aerobic zone. The anoxic fraction can be varied by adjusting the set point (Hartley, 1997).Fig. 2 shows the process-operating characteristic. There is an anoxic fraction-operating window that gives lowest total nitrogen (TN) in the effluent. This window shifts as operating conditions (feed and process operating characteristics) change. Lowest TN occurs where effluent ammonia and nitrate are about equal. The process can therefore be optimised in operation to maintain lowest (or any other) TN concentration. Because the mixed liquor recycle ratio is so high, the ammonia and nitrate concentrations are almost constant, and equal to the effluent concentrations, throughout the circuit.According to the UCT model, sludge settleability in an oxidation ditch should be best when TN is lowest, deteriorating at a lower anoxic fraction where the oxidised nitrogen concentration exiting the anoxic zone is elevated, and also at a higher anoxic fraction where the ammonia concentration increases. Actual SVIs in operating ditches are variable. For example, von Munch and Komarowski (2001) report an unstirred SVI value of 105 mL/g for the Thorneside plant and an sSVI value of 155 for the Gibson Island plant (both in Brisbane, Australia). At the Bundamba plant in the same region the unstirred SVI has been variable between 90 and 400 mL/g (Vivian and Hartley, 1989). Two Florida plants, Dale Mabry and Falkenburg, were reported to operate at unstirred SVIs of 140 and 180 mL/g, respectively (Hartley, 1995; Mines and Woods, 1994).This paper analyses the operating data from three Australian oxidation ditch plants to demonstrate that (a) the settleability characteristic does behave as expected, (b) the UCT model appears to require modification to explain the observations and (c) operating settleability can be readily controlled in the oxidation ditch process. The three plants are Bucasia (located in Mackay), Coolum and West Byron (Byron Bay), all treating domestic sewage. Physical and operating data are summarised in Table 1. Routine operating data have been used for the analysis and no information is available on the types of filamentous organisms present.控制氧化沟工艺中的污泥沉降K.J. Hartley摘 要本文介绍了一个针对控制硝化反硝化氧化沟活性污泥法中的污泥沉降这项新操作技术的开发的调查。据推测，具体的污泥容积指数（ SSVI ），最低的是在工艺缺氧分数最佳的部分，在较高和较低的部分污泥容积指数则会增加。采用出水氨氮：硝酸的比例替代缺氧分数，会发现，一个简单的基于三个固体的经验模型停留时间的移动氮比，能够完全复制SSVI长期变化的具有两个独立氧化沟的全规模工厂。从第二个氧化沟工厂的操作数据可以得出，不管前发酵槽是处于上线还是离线的状态，SSVI都会随RBCOD变化，RBCOD越高，SSVI越低。得出的结论是，最好的沉降发生在出水氨氮和硝酸盐大约相等的地方，出水总氮最低处。最好的操作规则是使用溶解氧控制，保持出水氨氮和硝酸盐氮的浓度大约相等。特意用这种方式操作第三个氧化沟厂，实现了，15个月的中位运行值为：SSVI为60mg/L，流出氨、硝酸盐和TN分别为0.2、0.3和2.0mgN/ L。关键词沉降；污泥容积指数；SVI ；控制 ；氮 ；氧化沟1 介绍氧化沟工艺由于其特殊的操作特性的能力，基本能够满足低总氮标准。氮的去除可以通过改变缺氧和好氧质量分数不断优化操作，以满足废水水量变化大的特点（Hartley, 1997; Mines and Woods, 1994）。还能观察到，在氧化沟中污泥沉降会随着操作质量分数的变化而变化而变化(Vivian and Hartley, 1989)。污泥沉降是活性污泥工艺中最重要的设计和运行特征之一。它会影响到池容的大小和实际处理能力。然而，活性污泥工艺被发明了近100年后(Ardern and Lockett, 1914)，对参数的准确控制无法得到求证。四个略有不同的测试了沉降比、污泥容积指数、搅拌的SVI、稀释SVI和SSVI3.5、或者仅是SSVI。这些参数的值与污泥的沉降性有密切的关系，它们互相之间的联系也可通过出版的统计数据看出。一系列影响活性污泥法中污泥沉降的因素，包括填料质量，固体停留时间（SRT ） ，混合特性，氧气状态， pH值和养分的有效性。对于氮和氮磷的清除过程中，开普敦大学的水研究小组已经进行了全面的沉降研究，他们Ekama等（1996）、卡西等人（1999）和Tsai等人（2003）发表了一系列论文和总结。他们的研究包括间歇曝气格式模仿氧化沟工艺。他们发现，在长污泥龄脱氮除磷工艺中低的污泥负荷组合丝状菌会导致几乎所有的膨胀问题，并且这个问题无法通过厌氧、缺氧和好氧选择器来控制。不管处理工艺有没有生物除磷，它都是通过控制氮的状态来控制污泥沉降。根据他们开发的模型，在二级处理A/O处理系统中主要利用缓慢生物降解COD，如果反硝化作用在缺氧区反应不完全并且出现了亚硝酸盐，那就说明在好氧区的细胞中还积累着NO且存在抑制反硝化的絮凝剂。丝状微生物，在缺氧区没有竞争力，因为他们只能反硝化硝酸盐为亚硝酸盐，它们能在好氧区增殖不受抑制。当COD被生物降解时，细胞内的NO不会累积，所以，RBCOD的一个显著功能就是减轻了絮