Management of Liquid Solid Waste

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Table 2. Results of Physico- Chemical Treatment of Leachate pH Adjustment and Aeration

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Table 3. Results of phosphoric acid treatment with aeration (air/water ratio 50:1)

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2.1. Physico-Chemical Treatment

A1L samples of leachate were aerated using laboratory aerators supplying 1-5 L of air per minute. Some aerated samples were also treated with sodium hydroxide, lime or phosphoric acid. During initial tests the impact of an air to water ratio on ammonia nitrogen and COD removal was investigated. It has been found that air to water ratio 50:1 resulted in an optimum reduction of both paramaters. For higher air to water ratios no significant differences were observed. Changes in the leachate chemistry after sodium hydroxide, lime or phosphoric acid treatment were also determined (Table 2 and Table 3). Tests with the leachate treated with sodium hydroxide (pH = 9.1), lime (pH = 9.3) or phosphoric acid (50 mg L^-1) were conducted at an air to water ratio 50 : 1. Control samples (without chemicals addition) were aerated in the same conditions as the treated samples. Treated and aerated samples were analyzed for COD, TOC, ammonia and suspended solids. Four tests were conducted for each treatment option and the results were then averaged.

2.2. Respirometric Tests

Respirometric tests were conducted using a Comput-OX, 12 channel respirometer (N-CON, Systems Company, Larchmon, N.Y.) equipped with a water bath with a precision temperature control. The system was monitored and controlled by a computer with CTOX program. In each respirometric run six reactors contained raw leachate and six others contained treated leachate. All reactors were fed with 2 mL of return activated sludge (RAS) taken from a wastewater treatment plant that was capable of degrading a variety of industrial wastes as well as providing nitrification. Respirometric tests were supported by chemical analyses of tested samples for the concentration of ammonia, COD, TOC, TSS and VSS. For raw and treated leachate three respirometric tests were conducted and the results were averaged.

Figure 1. Experimental arrangement of leachate

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Table 4. Results of Bench Scale Activated Sludge Studies

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2.3. Bench Scale Activated Sludge Studies

The bench scale treatment system (Figure 1) consisted of the following utilities: a leachate tank (1), a peristaltic pump (2), an aeration tank with air diffusers (3), a primary clarifier (4) a denitrification tank equipped with a magnetic mixer (5) an aerobic tank with air diffusers (6) a secondary clarifier (7), an ozonizer (8) an effluent tank (9) and a sludge storage tank (10). The system operated in continuous mode. Initially, the aeration tank was filled with a mixture (5:1) of treated leachate and RAS from the treatment plant and aerated for 4 days to acclimatize the sludge. The treated leachate was then pumped into the aeration tank at a rate 2.5 ml/min to maintain hydraulic retention time (HRT) of 100 hrs. Samples of the influent, the mixture from the aeration tank and the effluent were analyzed for pH, ammonia, nitrate, COD and TOC. The results are presented in Table 4. After 20 days the flow rate was reduced to increase HRT to 120 hrs and the denitrification unit was included into the treatment train. Initially the HRT of the denitrification tank was 12 hours which was subsequently increased to 20 hours. The recirculation rate in the denitrification unit was maintained at 5 times of the influent flow. Analyses of the influent and the effluent from the anoxic – aerobic system are presented in Table 4.

2.4. Ozonation Studies

Ozonation of the effluent from the biological system was conducted using a laboratory ozonator, generating 10 mg of ozone per min. 1L samples of the effluent were purged with ozone for 20 minutes. Treated samples, after 1, 2, 5, 7, 10, 15 and 20 minutes of ozonation were analyzed for COD, TOC and color. Five ozonation tests were conducted. The averaged results of 6 independent tests are presented in Table 5.

Table 5. Ozonation of the Effluent from Biological Treatment

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3. Results and Discussion

3.1. Leachate Characteristics

The investigated leachate, as presented in Table 1, has a very high concentration of organic substances (COD = 7880 mg L^-1; TOC = 2360 mg L^-1). Most of them is biodegradable (BOD = 4920 mg L^-1). The main inorganic constituents of the leachate are: bicarbonates (alkalinity = 3850 mg CaCO₃L^-1) and chlorides (1390 mg L^-1) of calcium (up to 530 mg L^-1), sodium ( 1310 mg L^-1) and potassium (up to 720 mg L^-1). Elevated concentrations of ammonia nitrogen (590 mg L^-1) and iron (63 mg L^-1) were also found. A high concentration of inorganic salts as well as organic substances in the leachate indicate complicated equilibria existing between cations and anions. Thus we can expect that the majority of calcium, magnesium and iron exists in the form of complexes with various ligands and not as a free cations. This had to be taken under consideration when design an effective treatment system. In order to meet stringent criteria for the effluent (BOD₅ = 10 mg L^-1 ; COD = 50 mg L^-1; NH₃-N = 1.0 mg L^-1; iron = 1.0 mg L^-1) very efficient treatment technology was required.

3.2. Laboratory Studies on Physico-Chemical Treatment

The aim of these laboratory studies on physico-chemical treatment of the leachate was to determine the ability to remove inorganic salts, organic substances and ammonia nitrogen using chemical precipitation and aeration. Initial aeration tests determined that an optimum air to leachate ratio is approximately 50:1 that is why this ratio was maintained during all subsequent aeration tests. Titration tests revealed a very high buffering capacity of the leachate, particularly above pH = 9.0. Therefore, pH adjustment tests were conducted only to this value which is much lower than required for efficient ammonia stripping (pH ~ 11). Results of these tests, as presented in Table 2 indicate that aeration of the leachate can substantially reduce the concentration of iron (64.9 %), organics (COD: 32.2 %; TOC: 29.8 %) and ammonia nitrogen (18 %). Alkalization of the leachate considerably enhanced the removal of iron (95.8-96.9 %) and organics (COD: 34.5-35.9 %; TOC: 30.2-32.6 %). The unexpected higher concentrations of ammonia in the alkalized leachate, as compared to that without alkalization is likely related to hydrolysis of organic subtances, at higher pH, which resulted in the production of ammonia. A decrease (10 %) of Total Kjeldahl Nitrogen in the leachate after aeration of alkalized samples confirmed this hypothesis.

Two sets of tests were conducted to determine the removal of ammonia nitrogen and magnesium from the leachate in the form of magnesium ammonium phosphate (MgNH₄PO₄). In one set of tests phosphoric acid was added to the aerated leachate. In the other tests aerated leachate was treated with phosphoric acid during mechanical mixing. Treated samples were analyzed for COD, TOC, iron and ammonia nitrogen.

Results of phosphoric acid treatment, as shown in Table 3, indicate that addition of phosphoric acid results in the additional removal of ammonia nitrogen (up to 44 % of the initial concentration). The method of phosphoric acid addition (before or after aeration) did not affect the removal of ammonia nitrogen or organics. The optimum dose of phosphoric acid was found to be about 50 mg L^-1. Higher doses of acid only marginally increased ammonia nitrogen removal but decreased iron removal. This phenomenon is likely related to fact that phosphate addition increases buffering capacity of the leachate and may enhance formation of soluble iron-polyphosphate complexes. Thus, pH of the aerated leachate treated with phosphoric acid is lower as compared to the raw leachate. At the same time the oxidation of iron (II) to iron (III) would be less efficient, since polyphosphates would stabilized iron (II). There was also a concern that higher concentration of phosphates in the pretreated leachate may exceed the biological demand and eventually might increase the concentration of phosphorus in the effluent above the discharge limit (0.3 mg L^-1). This would consequently necessitate additional treatment. The results of the physico-chemical treatment studies demonstrate that pretreating of the leachate through aeration and phosphoric acid treatment results is efficient. It would allow substantially remove of ammonia (up to 40 %) and organics (up to 30 % as COD and TOC). Aeration of the leachate without phosphoric acid or at pH ~ 9.0 was found less efficient in ammonia removal since elevated pH enhances hydrolysis of organic matter and generation of ammonia.

3.3. Biological Studies

The purpose of conducting biological studies was to determine the biokinetics of leachate treatment and the factors affecting organic and ammonia nitrogen removal. Biological studies consisted of respirometric and bench scale investigations. In order to determine the effect of pretreatment (aeration with phosphoric acid and sedimentation) parallel tests for raw and pretreated leachate were conducted. In order to simulate the conditions at the site, respirometric tests were performed in a water bath at 10°C. The results of respiromeric tests demonstrated a high biological activity (high oxygen uptake) of the biomass in the leachate. Substantial removal of COD (70 %) and ammonia nitrogen (65 %) was observed during exponential growth. Total suspended solids (TSS = 196 mg L-1) were substantially higher in reactors fed with raw leachate than those fed with pretreated leachate (TSS = 168 mg L-1), while volatile suspended solids (VSS = 136 mg L-1) were significantly higher in reactors with pretreated leachate as compared to those with raw leachate (VSS = 115 mg L-1). This phenomenon is related to the precipitation of inorganic solids, particularly calcium and iron compounds during biological treatment. Precipitation of solids in pretreated samples was less extensive since substantial amounts were removed during pretreatment. Higher VSS for pretreated leachate suggests that pretreatment removes some inhibitors of biological growth.

Biokinetic parameters were calculated using the IAWQ Activated Sludge Model No.1 ^{[15, 16]}. Non-linear regression analyses of the respirometric data were performed using the computer program MathCad (version 5.0 plus). These analyses indicated that the kinetics of the investigated biological system comply with the Haldane model for inhibitory wastes. Biokinetic constants, particularly maximum specific growth rate (μmax) and inhibition constant (Ki), were substantially higher for the treated leachate (μmax = 0. 138 h-1; Ki = 1048 mgCOD L-1) as compared to the raw leachate (μmax = 0. 128 h-1; Ki = 679 h-1). This indicates that pretreatment reduces the concentration of inhibitory substances in the leachate. Since pretreatment also removes a substantial organic load from the leachate it would allow one to reduce the size of the biological treatment system.

Bench scale activated sludge studies were conducted using the system presented in Figure 1. The system was fed with a raw leachate. The conditions in the system were monitored using a pH meter (influent, anoxic tank and aerobic tank) Redox probe (anoxic tank and aerobic tank) and DO meter (anoxic tank and aerobic tank). Samples of the raw and treated leachate were collected regularly and analyzed for COD, TOC, ammonia nitrogen, nitrites, nitrates, iron, color and turbidity. Samples of mixed liquor from the anoxic and aerobic tanks were collected twice a day and examined under a microscope. Initially, pretreated leachate (after aeration with 50 mg L^-1of phosphoric acid and separation of solids) was fed directly to the aeration tank. After 20 days the denitrification unit was included and the leachate was pumped into the anoxic tank, which had ORP ~ - 100 mV. The retention time and recirculation of RAS from the secondary clarifier into the aeration tank as well as recirculation of a mixed liquor from the aeration tank into the denitrification tank were adjusted during the studies to achieve the best removal of COD, ammonia and nitrates. The final parameters of the treatment system were:

Influent flow rate: 2.2 mL min^-1

Denitrification tank : HRT = 20 hours; MLVSS = 3800 - 4200 mg L^-1

Recirculation of mixed liquor: 10 :1

Aerobic tank: HRT = 100 hours; MLVSS = 4000 - 4600 mg L^-1

Sludge age: 11 days

RAS wasted: ~ 150 mL per day

During 30 days of operation at the above conditions, the system achieved a very high removal of ammonia nitrogen (average ~99.6 %) and organics (92% COD and 94 % DOC) as it is presented in Table 4. Microscopic examination of the biomass revealed rich and diversified microbiological life. The pH of the mixed liquor in anoxic and aerobic tank was also very stable and no pH adjustment was necessary. Despite very good efficiency of the biological system, the quality of the effluent was found unsatisfactory for discharge into the surface water, due to COD ~ 200 mg L^-1; color > 250 St Units and turbidity > 200 NTU. Therefore ozonation studies on biological effluent were conducted.

3.4. Ozonation Studies

The aim of the ozonation studies was to determine the ozone dose required removing the color from the leachate and improving settling of suspended solids. It was also expected that ozonation would reduce the COD load. Ozonation was conducted using a laboratory ozone generator, which was supplied with dried air. The effluent from biological system was treated with various doses of ozone and samples were analyzed for COD, DOC, color and turbidity. In addition, samples of ozonated effluent were treated with alum and polymer to study the effect of ozone treatment on the settling of suspended solids. As shown in Table 5, ozonation substantially improved the quality of the biological effluent, particularly when this treatment was followed by coagulation/flocculation. The optimum required dose of ozone was found to be 50 ÷60 mg O₃ L^-1. Higher doses of ozone only insignificantly decreased COD concentration, but did not affect color and turbidity.

Table 6. Total Performance of the Treatment System

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