Municipal Solid Waste Management involves a development of an insight into the impact of waste generation, collection, transportation and disposal methods adopted by a society on the environment and adoption of new methods to reduce this impact. MSWM is becoming a challenge in many countries around the world. Population growth, rapid urbanization and change in lifestyle are contributing to increase in municipal solid waste (MSW) generation. And its disposal is becoming a huge problem for the communities involved.
Sanitary landfill is one of the most widely employed methods for the disposal of municipal solid waste. After landfilling, solid waste undergoes physicochemical and biological changes. Consequently, the degradation of the organic fraction of the wastes in combination with percolating rainwater leads to the generation of a highly contaminated liquid called-leachate. Landfill leachate is a complicated and non-biodegradable wastewater, and it always contains high strength pollutants which have an adverse effect on the environment. It has been generally known as a high-strength wastewater that is most difficult to deal with. The common features of the leachates generated from landfill, are high strengths of organic concentration (chemical oxygen demand COD), ammonia (NH3-N), volatile fatty acids (VFA), heavy metals and other hazardous pollutants. These features are strongly depending on the age and type of solid wastes of a landfill site. Therefore, leachate has a high concentration of the pollutants and great changes in quality and quantity, which can pollute the surrounding environment and should be treated to meet relevant standards before discharge into sewers or direct disposal into surface water.
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In general, the appropriate leachate treatment methods are mainly based on specific characteristics of leachate under examination. The leachate treatment methods are physical, chemical, and biological.
When treating young (biodegradable) leachate, biological techniques can yield a reasonable treatment performance with respect to COD, NH3-N and heavy metals. However, when treating stabilized (low or non biodegradable) leachate, biological treatment may not be able to achieve the permitted maximum COD levels for direct or indirect discharge due to the recalcitrant characteristics of organic carbon in the leachate. Bio-refractory contaminants, contained mainly in older leachates, are not amenable to conventional biological processes, whereas the high ammonia content might also be inhibited to activated sludge microorganisms. As a result, the search for other effective and efficient technologies for the treatment of stabilized landfill leachate has intensified in recent years.
In more recent decades, a combination of physical, chemical and biological methods are often required for efficient treatment of leachate to meet local discharge standards. However, it is difficult to obtain satisfactory treatment efficiency by any one of these methods alone.
The leachate generated from the Chang Shankou landfill in Wuhan city (China) was characterized and submitted to a combined process in this study. Leachate samples from the pond on landfill site were collected and analyzed to investigate the main parameters of leachate characteristics. It was observed that this leachate presents a relatively high value of pH ranging from 7.80 to 8.60. It can be deduced that leachate corresponded to the methanogenic phase of decomposition. The COD, Biological Oxygen Demand (BOD5), NH3-N and Total Organic Carbon (TOC) concentration values were 2600-5000, 400-1100, 2200-3100 and 580-940 mg/L, respectively. The composition of landfill leachate is characterized as a low C/N ratio, low BOD5/COD ratio (0.15 to 0.20), and high contents of NH3-N, which show the leachate can be classified as "old" and non-biodegradable. This indicates clearly that it is difficult to remove ammonia and COD at the same process; therefore, pretreatment is required prior to biological treatment.
The present study can be divided into three parts: the first part research on suitable pretreatment process for non biodegradable leachat can employee and used on site. The main aims of this part are to reduce pollutant loading and enhance biodegradability of leachate before biological treatment or discharge leachate to sewer if it treated with domestic sewage in wastewater treatment plant. While in the second and third part a completed treatment process by a combination of physicochemical and biological processes was investigated. These combined processes aim to remove the crucial pollutants (COD and ammonia) from the leachate so that the water quality of the final effluent could meet the Chinese landfill pollution control standard (GB16889-2008.
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Ammonia stripping (air stripping coupled with agitation) as a modified process followed by coagulation-flocculation processes developed as a pretreatment process of non biodegradable leachate with initial concentration of 2800 mg/L COD, 2600 mg/L NH3-N and 560 mg/L BOD5. The main aims of these processes are reducing a concentration of NH3-N and organic matter as well as enhancing the biodegradability of landfill leachate. Ammonia stripped by air stripping under conditions of the airflow rate of 10 L min-1 at pH 11 for 3 h, while the agitation process applied to air stripping effluent for 2 h at the pH of 11.5 in 150 sec-1 gradient velocity. NH3-N was removed at 96% as removal ratio by the modified ammonia stripping in 5 h total stripping time. Ferric sulfate, poly ferric sulfate (PFS) and aluminum poly chloride (PAC) was tested as a coagulant material in the coagulation process. Chemical oxygen demand (COD), suspended solids (SS), turbidity as well as the sludge ratios were discovered for each material operated under optimum condition of pH and dosage.
For PFS the best removal ratios of COD, suspended solids (SS) and turbidity were 68%, 92% and 85%, respectively obtained at 1500 mg Lâˆ’1, while by using 1200 mg Lâˆ’1 ferric sulfate the removal ratios were 64%, 86% and 82%, respectively. Whereas 1500 mg Lâˆ’1 of PAC gives removal ratio 36%, 82% 78%, respectively. The Optimum pH was 5 for all of the materials. The generated sludge ratios for each material operated under optimum condition of pH and dosage were 4%, 5% and 9% from wet volume for PFS, ferric sulfate and PAC, respectively.
The overall removal of NH3-N, COD, Biochemical Oxygen Demand (BOD5), Total Organic Carbon (TOC), and SS obtained by these pretreatment processes were 96.5%, 71.5%, 56.5%, 48.5% and 96.5%, respectively at the corresponding biodegradable ratio was modified from 0.20 to 0.31.
The complete treatment of non biodegradable landfill leachate after the pretreatment process was carried out by two options. The first option by designed a laboratory scale of treatment processes consist of agitation, coagulation, sequencing batch reactor (SBR) and filtration process. While the second option consisted of agitation, coagulation, Internal Circulation Up flow Sludge Blanket (ICUSB) reactor.
In the first option agitation as a novel method of stripping was used to overcome the ammonia toxicity to aerobic microorganisms. The NH3-N removal ratio was 94% obtained at 5 h stripping time at pH 11.5 and the Gradient velocity (G) of 150 sec-1. PFS coagulation followed stripping; COD and BOD5 were removed at 70.6% and 49.4%, respectively. Biodegradable ratio BOD/COD was improved from 0.18 to 0.31within this primary treatment. Thereafter the effluent was diluted with sewage at a different ratio before it subjected to the SBR treatment. Each SBR cycle consisted of the following phase: fill (0.5 h), anoxic react (2 h), aerobic react (6 h), anoxic react (2 h), settle (1h), and finally draw phase (0.5 h). Up to 93.3% BOD5, 95.5% COD and 98.1% NH3-N removal were achieved by SBR treatment. A dual filter consists of carbon (0.2 Ì¶ 0.45) mm and sand (0.2 Ì¶ 0.45) mm used as a polishing process, the final effluent concentration of COD, BOD5, SS and NH3-N were 72.4 mg L-1, 22.8 mg L-1, 24.2 mg L-1 and 18.4 mg L-1, respectively. So that the water quality of the final effluent could meet the Chinese landfill pollution control standard (GB16889-2008).
In the second option ICUSB reactor employed for the removal of biological nutrient from leachate with initial concentrations COD 2800 mg/L, NH3-N 2200 mg/L, BOD5 420 mg/L and TOC 820 mg/L. Leachate was subjected to the stripping and coagulation as a pretreatment process to reduce both of the ammonia toxicity to microorganisms and organic concentration. The concentration of COD, NH3-N, BOD5 and TOC at the end of the pretreatment processes were 860.5 mg L-1, 172 mg L-1, 320 mg L-1 and 440 mg L-1, respectively.
ICUSB reactor is a set of anaerobic-anoxic-aerobic (A2/O) bioreactor which develops on the basis of expended granular sludge blanket (EGSB), granular sequencing batch reactor (GSBR) and intermittent cycle extended aeration system (ICEAS). The reactor was operated under three different operating systems consisting of recycling sludge with air (A2/O), recycling sludge without air (low oxygen) and combined both of them (A2/O and low oxygen). The lowest effluent nutrient levels were realized by using the combined of A2/O and low oxygen operation which resulted in effluent of COD, NH3-N and BOD5 concentration of 98.2, 13.5 and 22.5 mg L-1, which all of them below the permissible limit of the local discharge standard (GB16889 _ 2008).
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The optimal operating conditions for the removal efficiencies of the biological nutrient by using ICUSB reactor were conducted to evaluate the influence of parameters. The results showed that the average removal efficiencies of COD and NH3-N was achieved to be 96.49% and 99.39%, respectively under the condition of hydraulic retention time (HRT) of 12 h including 4 h of pumping air into the reactor, dissolved oxygen (DO) in an aeration period of 4 mg L-1 and up flow velocity (u) 2 m h-1. These combined processes were successfully employed and very effectively decreased pollutant loading.
Consequently, these options of two combined processes were successfully employed and very effectively decreased pollutants loading. The final effluent of the combined treatment could be directly discharged into waterway without effects on the health of aquatic ecosystems or considered for non-potable use. It is observed that these two combinations of leachate treatment are demonstrated outstanding treatment performances compared with other similar combined treatment in different selected countries. It is also concluded that the two treatment processes have perfect treatment effects making these methods are a competitive and an attractive economic alternative treatment methods.
Keywords: Landfill leachate; combined treatment; Ammonia stripping; Coagulation; SBR; A2/O bioreactor; Nutrient removal.