PILOT-SCALE MODELS OF TREATMENT OF LANDFILL LEACHATES COMBINED WITH URBAN W ASTEW ATERS IN A FACULTATIVE LAGOON

This study set out to determ ine the potential for treating leachates in combination with wastewater at Facu ltat ive Lagoons, a dev ice normally used for treat ing raw wastewaters. Pi lot-scale models were used to s imulate leachate treatment and d isposal in a Facultat ive Lagoon (FL), combining 2.4 L/hr of raw wastewater with a leachate m ixture (compris ing both young and old leachates), in concentrations of 4%, 6%, and 1 0% (v/v). The solution of leachate m ixture in raw wastewater was then fed into the two p i lot-scale mode ls ( M I and M2). The fol low ing parameters : concentrat ion of algae ; chlorophyl l a, b and c; faecal co l iforms (FC); and heavy metals, were analyzed in all the three component stages: the unmixed wastewater; the o ld and young leachate mixture ; and the combined wastewater­ leachate mixture. As a 1 0% di lution was found not to impede correct functioning of the Mode l Facultat ive Lagoon, the same concentrat ion was tried out using urban wastewaters from the University Wastewater Treatment Plant (UWTP). Interval values of BODs and COD in the wastewater and in the leachate mixture were 45-875 mg/L and 307-5 ,763 mg/L respectively, and results showed that a I 0% concentration of leachates combined with wastewater does not upset the system of biological treatment. None of the m ixtures affected the populat ion of algae. Maximum removal efficiency of BOD5 was 75%, and 35% for COD, therefore leav ing a BOD5 leve l of less than 25 mg/L in the efflouent from the FL. The removal of BOD5 and COD from the U WTP was greater sti l l , 86% and 64%, respective ly. FC and heavy metal concentrat ionso: As (0.007 mg/L), Cd (0.02 mg/L), Cu (<0.0 l mg/L), Cr ( 0.04 mg/L), Hg (o<0.00027 mg/L), Ni ( 0. 1 5 mg/L), Pb (0.098 mg/L) , CN (0.02 1 3 mg/L) and Zn (0.05 mg/L), were a l l be low the max imums establ ished by the Mexican Federal Regulat ion for Re-use of Wastewater in Agricultural Irr igat ion (NOM-ECOL-00 1 1 996).


Design of the pilot-scale model
Pilot-scale models were used to simulate leachate treatment and disposal in a Facultative Lagoon (FL), combining 2.4 L/hr of raw wastewater with a leachate mixture (comprising both young and old leachates), in concentrations of 4%, 6%, and I 0% (v/v). The solution of leachate mixture in raw wastewater was then fe d into the two pilot-scale mode ls (M I and M2). The fo llowing parameters : algae concentration; chlorophyl l a, b and c; faecal col iforms (FC); and heavy metals, were analyzed in all the three component stages : the unmixed wastewater; the mixture of old and young leachates; and the combined wastewater-leachate mixture.
The design of the model was based on the control of a laminar flow regimen. Each tank had drains that permitted the entrance and exit of water without provoking turbulence. Two identical models were constructed ( M I and M2): leachates were added to modea l M I , whi lst model M2 served as the control receiving only FL wastewater. Both models where the two treatments were carried out had tanks of 200 L capacity, and each was fitted with a system of continuous agitation. Figure I shows the characteristics of the pi lot-scale modea l as it was used during the study. To monitor progress, every 48 hours throughout the experiment samples of wastewater were taken for laboratory analysis from the Facultative Lagoons that lie about 1 .5 km from Texcoco Lake. Samples of leachate from the Bordo Poniente Landfil l were also taken. Each leachate sample was constituted i n the fol lowing way. Samples were taken from 1 2 points distributed around the Landfil l. These 1 2 samples were then mixed together giving one sample representative of the entire Landfi l l . Each sample-taking exercise was carried out in triplicate.

Figure I. Characteristics of the pilot-scale model.
The pilot-scale reactors were first filled with wastewater from the Churubusco Channel and inoculated with 60 L containing algae and sludge obtaine d from the Texcoco wastewater treatment plant. Fresh untreated wastewater was then fed in, ensuring the BODs and COD remained at 45 mg/L and 307 mg/L, respectively. The inflow was maintained at 2.4 L/hr, Kalmar ECO-TECH KALMAR, SWEDEN, November 26- 2 8, 2007 determ ined beforehand to be the ideal now volume. Frequent monitoring was carried out to ensure these parameters were maintained. Over the experimental period of 3 0-60 days, the percentage of BOD5 removal was found to remain constant. The reactors were therefore considered stabilized, and the process moved on to the next step.
Leachate was col lected from the lateral channels alongside each macro-cel l of the Bordo Poniente Landfill, and a leachate m ixture was created taking two parts by volume of stabilized leachate (old) with an average BOD 5 of 1 60 mg/L, plus one part by volume of young leachate with an average BOD 5 of 680 mg/L. This leachate m ixture was progressi vely introduced into the MI model, starting with 4%, fol lowed by 6% and then I 0% by volume (v/v) of leachate to wastewater in phases I, II, and I l l , respectively.
As the I 0% dilution was found not to impede correct functioning of the Modeo l Facultative Lagoon, the same concentration was tried out using urban wastewaters from the University Wastewater Treatment Plant ( U WTP). and m icro-organisms ( faecal col i forms, algae, chlorophyl l a, b and c).

Evaluation of the Operation of Both Models ( Pilot-Scale)
Operation of both models, M I and M2, was care fully observed both at start-up and during the actual treatment process. Particul ar attention was paid too : a) How the system functioned with regard to the most important variables of the treatment process, i .e. BODs and COD b) Because the Facultat ive Lagoon system functions due to the presence of algae, specific studies were carried out to determine the d iversity and quantity of algae in the experimental models. Chlorophyl l determination was made us ing the Azov spectrophotometer method [9], and the algae count involved the Sedgwick-Rafter technique [2]. c) Faecal col iform totals were calculated using the Most Probable Number ( MPN ), and Membrane F iltration ( MF) methods d) D issolved Oxygen was analyzed using the electrometric method ( Y S I ), the values taken in the aerobic zone of the models e) Heavy metals were analyzed with the atomic absorption method (EPA 60 l 0B), using an Inductively Coupled Plasma Atomic Em ission Spectroscope f) The pH was monitored using a potentiometer (Coler-Palmer, Model 05669-20) g) The final analysis made was of how ammonia is eliminated, or disappears, from the leachates.

Results of the physical-chem ical characte rization and m ic robiology of wastewater and leachate sam ples from the FL and the UWTP
The results of the physical-chemical characterization of metal s and microorganisms in the leachate and influent taken from the FL and UWTP are shown in Table I Table 2 shows the results of the physical-chemical quality, microbiology and metals present in the leachate-wastewater mixture at 4%, 6% and 1 0%. These results illustrate the significant decrease in the leachate heavy metal content when mixed with raw wastewater from the FL and U WTP. The leachates contained Chromium 2.4 mg/L; whereas the combined leachate-FL wastewater mixture gave Chromium 0. 1 05, 0. 1 5, 0.04 mg/L at 4%, 6% and 1 0% v/v, respectively. As the concentrations of this metal did not interfere with the activity of the microbial community present in the system, the same Table 2 gives the following average results at the three dilutions: faecal coliforms at 1 .5 X I 0 2 , 2 . 1 X 1 0 2 , 1 .2X 1 0 2 (MPN/1 00 mL); concentrations of algae fluctuating from 0.5 X I O J to 1 .8 X I 0 4 algae/m; and chlorophyl l a at  26 1 .57, 1 23 .36, 33. 1 (mg/m l )   In the case of micro-organisms such as faecal coli forms, in the stabilization period bacteria concentration registered 1 0 4 M PN/ 1 00 mL in the influent and 1 0 3 M PN/ 1 00 mL in the effluent, which demonstrates a logarithm in the rate of removal in the cellular concentration of these bacteria. These values persisted throughout the 60 days.

Results of B0D 5 and COD behaviour
The raw wastewater used evidenced a low 80D 5 concentration of 45 mg/L. Thi s is due to the significant length of open sewer along which it travels before reaching the Texcoco wastewater plant, during whi ch time part of the organic matter is transformed. However, at all experimental levels (after 60 days), with leachate mixtures of 4%, 6% and I 0%, overall BOD 5 removal efficiencies were 7 1 , 84 and I I O mg/L respectively. As a result of this low organic load, as shown in Figure 2

Figure 2. Residual BOD5 during the Stabilization of the Models and al each stage of the study: Ml with leachates and M2 without leachates.
COD values increased from 2 1 9 mg/L to 7 1 1 mg/L with the addition of leachates in a 1 0% solution (see Figure 3). However models M I and M2 maintained a steady level of COD removal during the whole experiment period, in other words, with and without the contribution of leachates. Over a period of 60 days, the pilot models demonstrated a constant COD removal of 35%. In a series of tests using wastewater typical of the UWTP, the average result showed greater removal of COD at 64%.

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Kalmar "cl" 6c. Algae are desirable in Lagoons as they generate the oxygen needed by bacteria for waste stabilization.

The types of algae present in the FL and in models MI and M2 were: Micros is/is sp. , Merismopedia sp. , Euglena sp., Scenedesm us sp. , Chlorella, Diatomeas and
Chlorophyll a is considered a good chemical indicator of phytoplankton biomass. It helps determine the trophic state of surface waters, and determines water quality. Chlorophyll a also allows detection of the adverse effects of pollutants on plankton. Despite its variability. chlorophyll a is one of the most frequently measured parameters in production studies [ 1 1 , 1 2].
Regarding chlorophyll b in the leachate mixture at 4%, 6% and I 0%, Figure 4 shows there were no significant differences between model M I and the M2 control model. This indicates that chlorophyll b concentration in the M I experimental model to which leachate was added, remained the same as in the M2 control model to which no leachate was added.
Chlorophyll c behaviour was very similar to that of chlorophyll b when 4% and 6% leachate mixtures were prepared. However, when model M I was operating with a I 0% leachate mixture, measurements showed that control model M2 evidenced slightly higher levels of chlorophyll c, which follows the pattern found with chlorophyll a.

Behaviour of faecal coliform
The levels of faecal coli forms (see Figure 5)

Behaviour of dissolved oxygen
In the case of dissolved oxygen, no di fferences were observed in any of the experimental phases ( 4%, 6%, or I 0%) when the leachate was incorporated. The values of dissolved oxygen in the aerobic zone of the models remained between I and 2 mg/L , which ensured appropriate conditions for metabolism of the system's micro-organisms; and the concentrations of dissolved oxygen (DO) at the surface remained above 2.0 mg/L.  Some studies show that metals such as Zn + z assist algae development, whereas Cr +6 can diminish the growth of algae and other micro-organisms, although the exact concentration able to inhibit growth has not yet been determined [ 1 1 ]. Concentrations of heavy metals in the Facultative Lagoon were remarkably low in all cases, and therefore, predictably, there would be no influence on algae development.

Behaviour of pH
The pH observed during all the tests remained remarkably stable, showing an alkaline tendency. Values ranged between 8. 1 and 9.3, with an average of 8.7 in both the effluents and influents. Table 3 shows the results of different operational conditions. A high percentage of waste removal for the 1 0% leachate per volume was observed in all cases with a m ixture of 2: I old to young leachates.
The removal of ammonia from leachates is an important treatment consideration. Water treatment plants are not designed to achieve this by nitrification, but rather by the uptake by bacteria of N itrogen, which is then incorporated into the biomass and removed as sludge. Results demonstrate that ammonia levels in water treatment plant effluents are consistently low (see Table 3). In Facultative Lagoons, the ammonia is incorporated into new algae biomass. Phosphorus removal occurs as sedimentation in the form of organic P in the algae biomass, or as precipitation at pH levels above 9.5. As this study gave pH levels ranging from 8.6 to 8.9 with the addition of leachate, there was no signi ficant change in Phosphorus levels. This, however, is positive, because Phosphorus is beneficial for agricultural irrigation.