| Cost is the key factor affecting wastewater treatments in China, and therefore the development of new techniques with objectives to reduce construction and operation cost when at the same time enhance treatment efficiency has become the major task in related fields. Accounted for by their characteristics of high treatment efficiency and low energy consumption, constructed wetlands used for municipal sewage treatments have received great attention during recent years. Another important aspect of applying wetland techniques to protect water environment is that it meets the concept of sustainable development.The present thesis consists of two major parts:Part I is focused on the establishment of an integrated constructed wetland system used for municipal sewage treatment, and Part II is a feasibility study to investigate the potential use of constructed wetlands for treatment of wastewater from pig farms after pre-anaerobic treatment. The main results are summarized as follows:(1) Establishment of an integrated constructed wetland system for municipal sewage treatment. Based on the principle of effect amplification of combined treatment processes and the theory of ecological balance, the system was established with multiple functions. The integrated model consists of A) a biological regulation pool, B) an aerobic reactor, C) a sub-surface flow wetland and D) a surface flow wetland. Experimental results show that:①The system, with adjustable and controllable characters and ecological self-adaption mechanisms endurable to seasonal changes, is a relatively stable system with high treatment efficiency.②The system with multiple functions performs much better than any individual unit applied. Unit A is used for primary degradation of organic pollutants; unit B is designed not only for reduction of CODcr, TN and TP to lower levels but also for increase of the micro-organism concentration aiming at enhancement of the bio-digestion ability of the whole system; unit C is filled with proper portions of vermiculite and red soil, which, with high cation and anion adsorption capacities, can not only be used to remove N, P and different poisonous metal ions but also to maintain the treatment efficiency at high levels in winter seasons when the bio-activity of the integrated system is low.; and unit D, associated with Unit C, where plant and aquatic lives play important roles in uptake and bio-digestion, functions in stabilizing water quality before discharge.③With high removal rate of TN, TP and CODCr the integrated system gives a combined effect of "1+1>2". Combination of different treatment processes resulted in not only enhanced treatment efficiency but also strengthened sustainability of the system against seasonal changes.④Plant species introduced in the system play important roles in absorption and transformation of N and P. Results from analyses indicated that a) The content of N and P in plants also varies in plant tissues and as well with seasons. b) For large-sized terraneous plants (plants growing in subsurface wetland), the proportion of N and P is high in above-ground tissues, which can thus be readily removed by harvest. Phytoplanktonic plants (free-floating aquatic plants) have higher N and P content than that of terraneous plants. c) The contribution ratio of plant uptake to N and P removal is related to the influent load of the wastewater. Plant uptake of N and P is higher in lower influent-load system. The contribution ratio of plant uptake in surface flow wetland to N and P removal is thus greater than that in biological regulating pool and subsurface flow wetland.⑤Integrated system yields "three beneficial effects". The land use of the system is less than 2 m2/t, which is reduced by 2 times more than that of conventionally constructed wetland systems for achieving the same treatment efficiency, leading thus to a significant improvement in terms of cost-effectiveness. Accounted for by its satisfactory treatment efficiency, low energy consumption and reduced land use area, the established model can be well applied as a municipal sewage treatment system in protection of water environment and conservation of aquatic ecosystems.(2) Establishment of a bio-vermiculite bed subsurface flow wetland. A three-stage terrace dropping-flow process using bio-vermiculite as bed fillers is established with purposes in design to reduce land use area, avoid bed silting, increase oxygen level and raise the use value of wetland plants. The results show that:①The optimal treatment efficiency can be obtained at relatively high hydraulic load (1.0 m3/m2.d). The total removal rate of the system for various pollutants in the sewage is greater than 65% and in general the effluent quality satisfies the GB discharge standard classⅡ.②For the four tested plant species, the degree of facilitation in management follows the order:Typha latifolia L. Canna indica Linn> Pomoea aquatica Forsk> Typha latifolia L.③The three-stage terrace dropping-flow process increases the DO level in the bio-vermiculite-bed layers. With a falling height higher than 45 cm, the oxygen concentration in a single water drop is higher than 0.57 mg/L and the oxygen recovery rate of the one-stage drop reaches 75%, indicating that the designed process is effective in improving the oxygen condition in the subsurface flow wetland system.(3) Feasibility study on potential use of constructed wetlands for treatment of liquid waste from pig farms after pre-anaerobic treatment. Three selected plant species, Pomoea aquatica Forsk, Oryza sativa and Eichhornia crassipes (Mart.) Solms are the popular ones growing in the south areas widely used as folders in hoggeries. Pot experiments were conducted to test the effect of these plant species for treatment of liquid waste from rural pig farms:①The order of treatment efficiency in terms of removal rate for CODCr is:Eichhornia crassipes (Mart.) Solms>Oryza sativa>Pomoea aquatica Forsk>control; that for TP is:Eichhornia crassipes (Mart.) Solms>Pomoea aquatica Forsk>Oryza sativa>control; for NH4+-N:Eichhornia crassipes (Mart.) Solms>control>Oryza sativa> Pomoea aquatica Forsk; and for TN:control> Pomoea aquatica Forsk> Oryza sativa> Eichhornia crassipes (Mart.) Solms. The removal rate of NH4+-N and TN is relatively high in the control pot and the reason is that in the pot where there is no inhibiting effect of plants on algae growth, high amounts of algae emerge in the later phase period and raise thus the removal rate of NH4+-N and TN to higher levels.②As a circulation economy mode by applying this ecological treatment method using folder plants to treat liquid wastes from pig farms, a bio-mass/energy circulation chain is established and the final goal of this trial is to obtain substantial "zero" discharge under the condition that high economic benefit is further ensured. |
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