给水排水工程专业英语论文.doc

Water and Wastewater Engineering
Final Class Project
Title：Applications of municipal wastewater treatment in lives
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Applications of municipal wastewater treatment in lives
Abstract：This article describes the following sections：sequential combination of photocatalytic oxidation with constructed wetlands is the study and the experimental evaluation of an alternative and innovative wastewater treatment system， which combines the action of photocatalytic oxidation with the surface flow constructed wetlands.A new contact oxidation filtration separation integrated bioreactor was used to treat municipal wastewater。the synthetic polymers normally used in the coagulation—flocculation treatment of waste water requires sustainable alternatives。
Keywords:bioreactor;coagulation;flocculation；photocatalytic oxidation；
Combination of photocatalytic oxidation with constructed wetlands
Wastewater treatment systems have been designed to minimize the environmental impacts of discharging untreated wastewater.Different options for wastewater treatment have different performance characteristics and also different direct impacts on the environment. During the last decades centralized conventional wastewater treatment systems were typically provided to large cities and secondary towns。 They involve large capital investments and operating costs, resulting in systems which are considered as not proper solutions for small villages that cannot afford such expensive treatment facilities.In addition， low water use rates and several operational problems have been encountered with such systems in rural and especially in touristic areas where the population size varies seasonally. The need for alternative methods of wastewater treatment is of interest to regulating authorities everywhere.
The so-called advanced oxidation technologies （AOTs） are among the most effective chemical oxidation processes， and are currently gaining significant importance in water treatment applications。 Under this term the scientific community refers to the technologies whose effectiveness is based on the production of the •OH radicals, one of the most powerful oxidant reagents。They can easily attack the organic molecules leading to the production of organic peroxide radicals and their final conversion to CO2，H2O and inorganic salts. The increased concern for the use of the AOTs may be explained by the need for seeking of new，alternative to the conventional ones and environmentally friendly technologies。
Among these, heterogeneous and homogeneous solar photocatalytic detoxification methods （TiO2/H2O2,Fe3+/H2O2） have shown recently great promise for the treatment of industrial wastewater，groundwater and contaminated air， allowing the contribution of the renewable sources of energy （solar energy) to the process of cleaning and restoring the environment.
The system combines the action of photocatalytic oxidation with the surface flow constructed wetlands in order to utilize the high solar irradiation in the Mediterranean region and the ability of the constructed wetlands to improve water quality through natural processes, providing treated wastewater capable of being reused, e.g. for irrigation。 More specifically,the method consists of the combination of a common physicochemical precipitation， a solar photocatalytic reduction of the organic content of wastewater by the use of a heterogeneous or homogeneous solar photocatalytic method, and finally passage of the wastewater through a constructed wetland， for the final purification of the wastewater to make its reuse possible.
A contact oxidation filtration separation integrated bioreactor
Traditional wastewater treatment processes are usually equipped with different treatment units that have a variety of functions to achieve acceptable treatment performance. Conventional processes for municipal wastewater treatment have disadvantages such as complicated operation, high running cost, excessive installation and large land requirement。 As a result， wastewater treatment plants normal operation face a number of key challenges in developing countries such as China。
In comparison with those traditional wastewater treatment processes, bio—film systems have low land requirement and flexible operation。 Bio—film technology systems include trickling filters， biological aerated filter （BAF）, rotating biological contactor （RBC）, fluidized bed reactor (FBR） and moving-bed reactor （MBR）.
To help overcome these issues， a new contact oxidation filtration separation integrated bioreactor (CFBR） was developed， which was composed of a Bio—film reactor (the upper part of CFBR) and a gravitational filtration bed prototype and principles of slow and cake filtration .Bio—film configuration coupled with gravitational filtration bed combines the advantages of the Bio—film reactor and the filtration bed in one system. As the use of filtration process, the CFBR can keep its effluent SS at a lower level. Moreover， the low investment cost of CFBR allows a cost—saving advanced treatment for pollutants from small and medium sized factories as well as separate streams of process wastewater, especially in developing countries such as China。
In developing countries such as China, which a trade-off between environmental protection and economical operation has to be made based on the local socioeconomic situation， cost is more important than other factors when choosing a process. Construction and operation cost are two important factors in evaluating a wastewater treatment process. And 60—90 percent of operation cost is electrical power consumption in wastewater treatment. According to Lou’s study on power consumption of municipal wastewater process， irrespective of electrical power consumption of sludge digestion, the electrical power consumption of WWTPs surveyed is 0。21-0。36 kWh/m3 of wastewater treated in China during 2000-2004.As CFBR described， its construction cost is low due to a single reactor of the whole system. And electrical power consumption of CFBR system was 0。13-0。15 kWh/m3 of wastewater treated, and 1。37—1.50 kWh/kg BOD5 removals. Hence, compared with traditional secondary municipal wastewater treatment processes， the cost of CFBR was very low. The results could be explained in two following aspects。 On the one hand, there is no sludge return operation in CFBR process。 On the other hand， the aspect ratio （i。e. height—to-diameter ratio) of CFBR is 21，which has achieved good DO utilization ratio。 Therefore, CFBR is capable of good power consumption。
Coagulation-flocculation treatment of municipal wastewater
Coagulation—flocculation treatment is commonly used to reduce the turbidity of municipal and industrial waste water, the coagulants in question typically being divalent or trivalent metallic salts or polymers that have low solubility in the pH range used. Metal salts hydrolyze rapidly in waste water to form cationic species， which are adsorbed by negatively charged dirt particles,resulting in simultaneous surface charge reduction. Polymers have been used in the coagulation—flocculation process for decades to reduce coagulant dosages， the volume of sludge and the ionic load of the waste water （especially the level of aluminum）, and to save overall costs。
The polymers used in coagulation-flocculation treatment are commonly synthetic poly-acrylamides， poly-acrylic acids and poly-styrene sulphonic acids and their derivatives, which are not readily biodegradable. Moreover, they may also contain un-polymerized monomers and additives that are neurotoxic and carcinogenic. Also, synthetic polymers are produced from oil-based raw materials, which make them non-renewable chemicals。 Consequently， there has been a growing interest in replacing oil-based flocculants with more sustainable natural bio—based alternatives。
Many natural—based flocculants are environmentally friendly and biodegradable， with good flocculating ability. These include bio polymeric materials such as starch, guar gum, chitin, pectin and algin， and some derivatives of natural carbohydrates such as dextran and pullman have also been investigated for these properties. Derivatives of cellulose, which is the most abundant biopolymer on earth, are nevertheless still scarce。 One potential and environmentally sustainable method of producing cellulose flocculants is to introduce reactive aldehyde functionalities into cellulose by aqueous periodate oxidation， as has been reported earlier。 The aldehyde groups of 2,3-di—aldehyde cellulose (DAC) can easily be converted further and in a selective manner to various functional groups such as carboxylic acids， sulphonates or imines. Recently Liimatainen et al。have investigated the use of anionic （ADAC) and cationic cellulose （CDAC） derivatives in the flocculation of kaolin suspensions, with promising results， showing that anionic cellulose nanoparticles resulted in better flocculation performance than the corresponding fully water-soluble derivatives。 Also Hokkanen et al。used modified nanofibrillated celluloses to remove heavy metals from aqueous solutions with promising results.
In the present research aldehyde groups of DAC pulp fibres were converted to carboxylic acids and these anionic cellulose derivatives （DCC） were nanofibrillated with a homogenizer. The flocculation performance of five anionic dicarboxylic acid (DCC） nanocelluloses produced in this way with variable charge densities was examined in the context of the coagulation–flocculation treatment of municipal waste water. The effects of DCC dosage and pH on flocculation were studied by measuring residual turbidity and COD of the settled suspension and compared the results with the performance of a commercial coagulant （PIX 105 A) and a commercial combination of a coagulant and a synthetic polymeric flocculant。 In addition, the aldehyde and carboxyl content， charge density (CD）， size and stability of each DCC were determined in an aqueous solution.
References:
［1] A。 Antoniadis，V。 Takavakoglou。Municipal wastewater treatment by sequential combination of photocatalytic oxidation with constructed wetlands，Available online 18 April 2010。
[2］ Z。H.Li, K。Yang,Treatment of municipal wastewater using a contact oxidation filtration separation integrated bioreactor，Available online 26 February 2010.
[3] Terhi Suopajärvi, Henrikki Liimatainen，Coagulation—flocculation treatment of municipal wastewater based on anionized nanocelluloses，Available online 15 July 201.
[4］ Weiwei Mo， Qiong Zhang，Can municipal wastewater treatment systems be carbon neutral？Available online 8 September 2012.