The Pilot Experiment Study On UASB-Magnetic-Biology Combined Process For Domestic Sewage

Rapid urbanization in our country has led to very high population density, the increased amount of domestic and industrial wastewater, and the stressed accommodation capacity of the surrounding environment in urban areas. Recently, fresh and marine eutrophication has become a worldwide problem. The shortage of water resource is not only the quantity but also the quality. The N and P, which caused the eutrophication, comefrom point sources pollution-industrial and municipal wastewater and non-pointsources pollution-agricultural and pasture irrigation.At the present, aerobic processes are widely used for municipal wastewater treatment. However, aerobic processes have serious drawbacks, such as considerable investment, operation and maintenance costs and high sludge production quantity, etc. Due to the shortage of financial sources, in many cities of our country adequate municipal wastewater treatment plants cannot be constructed and operated. It is required to adopt appropriate approaches to remove oxygen consuming organic and nutrients from municipal wastewater. Therefore, it becomes more and more important to develop appropriate wastewater treatment processes, which will have high treatment efficiency with low capital cost, low maintenance cost, and simple operation requirements. A main objective is to set up adequate wastewater treatment systems, which are simple in design and efficient in removing the pollutants. Energy consumption in these systems should be low, reuse of water and valuable by-products should be the maximum. So can increase the treatment efficiency of wastewater and reduce the quantity of contamination.Anaerobic biological treatment technology, which has been developing quickly in the recent decades, seems to be an ideal and sustainable solution for water environment protection. Anaerobic treatment processes not only consume less energy, but also produce useful energy in the form of biogas. However, anaerobic processes have disadvantages, such as high bacterial sensitivity to some environmental conditions (mainly pH, temperature, and toxic compounds.), long starting duration, and the production of malodorous compounds, etc. The interest on anaerobic systems as the main biological step (secondary treatment) in wastewater treatment was scarce, only in high industrial wastewater, until the development of UASB reactor in the early 70s. Due to its perfect treatment effectiveness, the anaerobic treatment process has been widely used for thetreatment of several types of wastewater. It also provides another option for medium and low concentration wastewater treatment. Nevertheless, some characteristics of sewage, like low chemical oxygen demand (COD) concentration, relatively low temperature, can have a negative impact on the process performance. Domestic sewage treatment in UASB reactors has presented significant results in tropical and sub-tropical regions. One of the most important challenges is the adaptation of the UASB process to the treatment of low temperature and low-concentration wastewater in warm climate.The effluent from UASB reactors usually needs be further treated in order to remove remnant organic matter, nutrients and pathogens. Natural biological treatment technologies, which have high treatment efficiency with low capital cost, low maintenance cost, simple operation requirements, are rapidly developed for wastewater treatment. That is a simple and effective method for post-treatment of anaerobic reactor effluent. Therefore, it is expected that UASB followed by natural biological treatment process will have COD, TN, TP removal efficiencies as higher as 85%, 40% and 40%, respectively. It not only increases the efficiency of the wastewater and guarantees the cleanness of the production of the wastewater treatment, but also overcomes shortcomings of the anaerobic treatment.The thesis focuses on the pilot experiment and a kinetic process for treatment of municipal wastewater by a combination of UASB with natural biological treatment process. There are six parts in this thesis.Part 1 introduces the process development and analysis on status of the municipal wastewater treatment. This part systematically provides the description on the mechanism and current situation of anaerobic and natural biological treatment technology dealing with the domestic wastewater.Part 2 focuses on the pilot experiment for treatment of municipal wastewater by UASB. Some of the salient findings of the study are summarize as follows:(1) The start-up of reactor seeds with the granular sludge according to the required sludge-loading rate (SLR) of UASB reactor, and completes within 5 days at temperature about 20 °C. The sludge loading rate (SLR) or the volume loading rate (VLR) of the start-up is 0.1kgCOD/(kgVSS-d) and 3.4kgCOD/(m3-d), respectively. This optimal SLR in pilot test was different from that in small-scale test. There was different start-up SLR due to the different wastewater concentration and water temperature. The UASB system could not be operated satisfactorily, and steady granular sludge cannot be formed at low CODconcentration. Therefore, the SLR of the start-up would be lower when the sewage wastewater was low-concentration and low-temperature. Operation at lower loading rates will be without solids separation. The SLR must be more than 0.03kgCOD/( kgVSSd), and VLR less than 0.6kgCOD/(m -d), otherwise, a dense sludge bed would not be established in the bottom of the reactor. Therefore, it is obvious that the utmost concentration of the UASB reactor must be more than lOOmgCOD/L.(2) The optimal operating parameters of UASB reactor are SLR from 0.06 to 0.13kgCOD/ (kgVSS-d), upward liquid velocity from 1.4 to 1.7m/h, hydraulic retention time (HRT) from 2.5 to 3.Oh at temperature in the range from 25°C to 32°C, while the COD removal efficiency is from 60% to 80%.The height of the UASB reactor would be bigger while dealing with the lower concentration wastewater. In order to mix the wastewater with sludge, larger upward liquid velocity will be adopted, since there is less gas production while dealing with the low strength wastewater. At the same time, it is important to have the enough HRT.(3) There are some factors affecting the removal efficiency of UASB reactor, which are COD concentration, HRT, water temperature and alkalinity. The COD removal efficiency could be calculated directly by:77COD = 13.21+0.016X! + 3.966x2 +0.980x3 +0.029x4. HRT and water temperature are the primary factors.(4) The kinetic coefficient of the UASB are: K,=5.12mgCOD/L, vmax=3.91d"1, YT =0.1742VSS/mgCOD, Kd=0.0308d"\ at temperature about 20°C. The equation of thewater temperature and the reaction velocity is: K2 = 60.67e~2'5M0 IRT.(5) It can enhance the removal efficiency of COD about 8%, while the magnetic equipment is combined with UASB reactor.Part 3 studies on the pilot experiment for treatment of UASB effluent in a series of stabilization ponds and wetland system, which can reduce the concentration of COD, N, and P of the wastewater. Main conclusions derived from the study may be summarized as follows:(1) The removal efficiency of COD, TN and TP of the system is 77%, 38% and 33% representatively, while the HRT is 3 days, thus making the effluent concentration meet secondary treatment effluent standard of domestic wastewater.(2) HRT and water temperature are the main factors effecting removal efficiency ofthe natural system to contamination. The removal efficiency of COD, TN and TP could be calculated directly by follow equations:Vcod =8.80-0.039x1 +10.394x2 +0.981x37]m =-5.056-O.238XJ +4.139x2 +1.331x3fjjj. = -22.106 -1.469x, +73977x2 +1.151x3In addition, the coefficient of the Wehner-Wilhelm model can be obtained by calculating the experimentation data, which are K^pO.^ld"1 and 9=1.07.Part 4 discusses the small-scale experimentation of sewage irrigation with the effluent of the wastewater treatment system. Sewage irrigation can utilize the nitrogen and phosphorus as nutriment to meet the requirement of plant growth. On the one hand, sewage irrigation can reduce the contamination of the wastewater and enhance the water quality. On the other hand, sewage irrigation can decrease the quantity of fertilizer utilization and decrease the non-point sources pollution of agriculture irrigation.This experimentation performs in arenaceous soil where cabbage and grass are planted. The transfer rates of N and P in the soil are O.lmm/d and 0.075mm/d.In order to avoid ground water pollution, it is necessary to know the characteristic of soil and the depth of the ground water, before the wastewater for agriculture irrigation is utilized. At the same time, it should be cautious to select the species of plant to protect the public health and avoid the pollutant accumulation in the food cycle. In order to guarantee the safe of environment, it is more importance to constitute the particular policy of sewage irrigation.Part 5 discuss the small-scale experimentation of controlling the nitrogen and phosphorus strength of municipal wastewater with Spirulina platensis. The examination algae are Spirulina platensis 439 and 972. The 439 and 972 removal efficiencies of total nitrogen are from 61% to 72% and from 63% to 79%, respectively under spirulina platensis of 20Chlug/L, at temperature from 25°C to 32°C, for 6 days. At the same condition, the 439 and 972 removal efficiencies of total phosphorus are from 5.8% to 15% and from 3.1% to 10.6%, respectively, for 5 days.The main affecting factors were the water temperature and concentration of CODusing spirulina platensis to remove nitrogen and phosphorus. The removal efficiency ofTN would reduce to 20% when the water temperature was less than 10°C. The removalefficiency of TN would reduce to 40% when COD concentration was more than lOOmg/L.The removal efficiency of TN and TP under different TN or TP concentration,temperature, alga concentration and COD concentration, could be calculated directly by following equations:Spirulinaplatensis 439: 77TO = 9.732-1.726x,+3.226x2 +1.136x3 +0.423x4 -0.137x5 77TP =2.713 -6.971x,+0.988x2 +0.419x3 +0.204x4 -0.173x5■ Spirulina platensis 972: r/m = 8.703-1.897x1+3.581x2 +1.279x3 +0.329x4 -0.204x5 tJtj, = 2.537 -4.544x1+0.560x2+0.336x3+0.136x4-0.107x5Therefore, the way to exploit spirulina platensis to control TN and TP of sewage applies to the secondary efferent of sewage treatment whose concentration of COD was less than 50mg/L. This is a sound way to control the point sources pollution and decrease the fresh and marine eutrophication problem.Part 6 analyses the optimizing results of the combination of stabilization pond with wetland system and the multi-stabilization pond system by Genetic Algorithm. The HTR of anoxic pond, wetland and secondary anoxic pond are 6.16d, l.OOd and l.OOd, respectively, while the removal efficiency of COD, total nitrogen and total phosphorus are 85%, 40% and 40%, respectively. Accordingly, the HTR of prima anoxic pond, algae pond and secondary anoxic pond are 6.13d, l.OOd and l.OOd, respectively. At the same object, the HTR of prima anoxic pond, wetland and secondary anoxic pond are 4.96d, l.Old and l.OOd, respectively, while the wastewater treatment system is combined with magnetization equipment.It is important to combine the anaerobic treatment process with the natural biological treatment process in series, i.e. UASB reactor with stabilization ponds, to increase the efficiency and reduce the treatment cost. Namely, the wastewater treatment process combines the treatment process of the wastewater with the complex utilization process.