RESEARCH WORK

Efficiency of the treatment of pig production residues in covered lagoon digesters

D. Blanco¹, J. Suárez¹, J. Jiménez², F. González¹, L. M. Álvarez¹, Evelyn Cabeza¹ and J. Verde ¹

¹Estación Experimental de Pastos y Forrajes Indio Hatuey, Universidad de Matanzas, Ministerio de Educación Superior.
Central España Republicana, CP 44280, Matanzas, Cuba.
²Biosoluciones Granda, Mérida, Yucatán, México
E-mail: dayrom.blanco@ihatuey.cu

ABSTRACT

The efficiency of two covered lagoons, designed to treat the residues of the pig production farms P-3 and T-2.1 belonging to the Association of Pig Producers of Yucatán, México was evaluated, in order to verify the feasibility of implementing this technology in Cuba. The physical-chemical and microbiological indicators of the effluents were determined and the moment of their entrance and their removal from the digesters, and upon their exit from the stabilization lagoon. The digester of farm P-3 could remove more than 90 % of the chemical oxygen demand (COD) and up to 71 % of the total suspended solids (TSS) present; while the digester of farm T-2.1 reached a removal of 78 % of the COD and 62 % of TSS. The sanitary analyses indicated that the total coliform bacteria showed an important decrease, from 2,4 x 10⁸ to 1,7 x 10³ in farm P-3 and from 4,2 x 10⁷ to 2,7 x 10³ in farm T-2.1. In both lagoons, the helminth eggs showed a reduction of 100 %. It is concluded that the covered lagoons had an adequate performance in the treatment of pig production residues, and that this technology is feasible to be used in Cuba.

Keywords: Coliform bacteria, biogas, helminths.

The search for sustainable alternatives in the treatment of residues from intensive animal rearing constitutes a priority task worldwide (IEA, 2013).

In Cuba, diverse designs and technologies have been used for the construction of digesters. The most used are the fixed-dome, mobile-dome and polyethylene-tube digesters, all of them useful, but with the common limitation of not being effective for treating large volumes of residues (Oviedo, 2011; Guardado, 2013).

That is why state organisms such as the Pig Production Entrepreneurial Group, which has 132 production centers, as well as many private farmers, need to have digesters with capacity higher than 100 m³, according to the sizes of their herds (EEPF-IH and Cubaenergía, 2014).

As response to this problem, in the last years work has been done with digesters designed as covered lagoons, which use geomembrane structures and domes where the oxidation of organic matter and the retention of gases occur (Díaz and Vega, 2013).

Among the most used materials the high density polyethylene (HDPE) stands out, whose technical functioning is similar to that of other most costly polyethylenes, such as PVC and EPDM, for which it is an attractive option from the economic point of view.

Mexico is one of the countries that have installed an important number of large-scale biogas plants using HDPE as cover material, in order to solve the problem of the effluents of large concentrations of animals, especially those of pig production farms(Pérez and Brigadiel, 2011); and has accumulated experiences in the application of such technology.

For such reason the objective of this study was to evaluate the efficiency of two covered lagoons built with HDPE in two pig production farms in Yucatán, México, to verify the feasibility of implementing this technology in Cuba.

METHODOLOGY

The study was conducted in the pig production farms P-3 and T-2.1, belonging to the Association of Pig Production Farmers of Yucatán, Mexico, which maintain a population of 20 000 and 1 300 animals each and have a daily wastewater discharge of 1 167 and 76,8 m³, respectively.

In 2012, biogas digesters were established in these farms for the treatment of residues. For such purpose, lagoons were built in both farms, with the bottom covered by HDPE of 1,2 mm of thickness, with multiple entrances and agitation, and once installed they were covered with the same material.

As part of the system, each lagoon had a stabilization or discharge lagoon for the secondary treatment of residues; the dimensions of the digesters are shown in table 1.

Evaluation of the systems. Physical-chemical and microbiological measurements were conducted in samples of wastewaters which were collected on the same day in previously-determined sites of each system (fig. 1).

Analysis of the physical-chemical indicators. The sample taking of the residues was conducted according to the criteria expressed by Bartram and Rees (1995). They were doubly stored in 500-mL plastic flasks with caps; which remained under conditions of darkness, in iceboxes, until their transportation to the laboratory.

Five samplings were carried out, with weekly frequency. The samples were analyzed in the Laboratory for Wastewaters REPAMA S. C. P. in Merida, Yucatan, according to the techniques established in the methods for the analysis of water (APHA/AWWA/WEF, 2005).

In the measurements of the chemical oxygen demand (COD), the colorimetric method 5220-D of closed reflux was used, and in the pH measurements, the method of Part 2320-A was applied. The total suspended solids (TSS) were measured according to the description in Part 2540-D. The analyses of total nitrogen (TN) were conducted according to Parts 4500-B and 4500-E.

Microbiological indicators. The total coliform bacteria (TCB) were determined according to the standardized methods (APHA/AWWA/WEF) of Part 9000 of the Microbial Examination. To identify the helminth eggs the Mexican norm NMX-AA-113-SCFI was used, according to the Bailinger-OMS technique, approved by the World Health Organization (WHO, 1989).

Statistical analysis. The data were processed through sample inference for two independent samples, from a t-test, with a significance level of 0,05. For such purpose, the statistical pack InfoStat version 1.1 was used.

RESULTS AND DISCUSSION

The evolution of pH is an important element within the digestive performance of a reactor. In the evaluated systems an alkalization of the residues occurred during the process, with increase in the pH (fig. 2), without statistical differences between them. This change was partly due to the water hardness in the zone, but also to the production of carbonates in the process of anaerobic digestion of the organic matter (Rendón, 2007).

The values found in this analysis coincide with the ones obtained by Ruiz (2010), who stated that the anaerobic digestion that occurs in a digester has several stages, and in each one the microorganisms show their maximum activity within a differentiated pH range, which reaches between 6,5 and 7,5 in the methanogenic stage (final stage). On the other hand, Stams (2004) stated that an increase of pH at the end of the process indicates that the reactor efficiently converts the organic matter to volatile fatty acids and carbonates.

The COD is an indicator that represents indirectly the organic matter content of a residue (Morales and Moreno, 2004). In the farm P-3, the indicators related to the organic rate experienced a marked decrease after the passage of the residues through the digester, which could remove more than 90 % of the COD and up to 70,84 % of the TSS in the biofermentation process (table 2). In the farm T-2.1, although the numerical values were lower with regards to those of P-3, adequate removals were found, of 77,6 % for the COD and 62 % for the TSS.

These results suggest that the covered lagoons showed a better performance as the system had higher dimensions, although it is important to take into consideration that the lagoon of farm T-2.1 received a residue with higher concentration of TSS.

Both systems showed in general adequate results, although the studies conducted by Osorio et al. (2007) showed that in similar systems removals of biological oxygen demand (BOD), COD and TSS can be reached of 97,4 %; 96,1 % and 95,1 %, respectively, and pH values upon the outflow close to neutrality; which indicates their high efficiency, in biogas production as well as in removal of pollution load.

Noyola (1997) stated that such performance is due to the increase of sludge and methanogenic bacteria which, with time, are stored in the digester; and that the high removal of total solids is due to the fact that a large part of them are volatile solids and behave similarly to the raw material for the biogas production.

The chemical indicators of the residues also showed decrease in their concentrations, although not so markedly as the indicators of organic contamination. The nitrogen in P-3 decreased from 705 mg/L to 512 mg/L at the end of the treatment process. In T-2.1 a higher numerical decrease appeared: from 896 mg/L to 378 mg/L at the end of the treatment.

Similarly, Cubillos (2006) reported that well-functioning digesters develop during the digestion process a denitrification, in which the nitrate contributed by the aerobic reactor is transformed into molecular nitrogen from the bacterial action, and this process ends when the last traces of dissolved oxygen disappear and the heterophagous bacteria breathe the oxygen combined in nitrites and nitrates, releasing nitrogen molecules.

The removals of this nitrogen is very important, because the nitrates accumulated in the deposits of wastewaters can be filtered to the groundwater and enter the wells that are used for the supply of drinking water, with marked consequences in human and animal health (Paulson, 2014). On the other hand, the pouring of wastewater with high nutrient content (mainly nitrogen as nitrate, nitrite or ammonia) in aquatic ecosystems, originates a concrete problem of water contamination called eutrophication. As the nutrient availability increases, so does the primary photosynthetic production, which is mainly represented by the proliferation of microalgae (Claros, 2012).

However, one of the main advantages of the biogas technology is the value of its final effluent, to a certain extent because of the nitrogen compounds that are found in the sludge flow and which confer it a fertilizer effect. Fish and Russo (2012) stated that when the sludge is used in the agricultural productions as organic fertilizer in the form of fertigation, they acquire an important added value, because it allows to reincorporate to the soil the nutrients which are not utilized by pigs and that are left as part of the sludge used for the biogas production.

Mandujano (2001) reported that when a cubic meter of biofertilizer is produced and applied, up to 200 kg of N/ha are provided, from which between 60 and 70 kg will be available in the first year. An important advantage of this fertilizer is that it doesn't leave toxic residues in the soil, improves its quality and is capable of competing with chemical fertilizers or complementing their use (Chavarría, 2014).

This is an important element that should be taken into consideration to evaluate the technical and economic feasibility of digesters. In the recovery of the investment the sludge effluent should be valued, which can be used as organic fertilizer because of contributing nutritional elements that are useful to recover soils and fertilize crops (Suarez et al., 2011, 2014).

The evaluation of the sanitary indicators of the wastewaters showed that there was an important reduction in the population of total coliform bacteria in the different sampling spots, with optimum values after passing through the stabilization lagoon (table 3). This suggests that the combination of the anaerobic and aerobic stages to which this sludge is exposed contributes to eliminate pathogens (Cabirol et al., 2002).

In the farm P-3 the population of TCB could be reduced from 2,4 x 10⁸ to 1,7 x 10³ MPN/100mL, which represents a reduction of five decimal units; while in the farm T-2.1 the population decreased in four decimal units: from 4,2 x 10⁷ to 2,7 x 10³ MPN/100 mL. Such result, although numerically lower than that of P-3, did not differ statistically. This indicator showed an adequate functioning in both systems, especially if it is considered that the final value is within the one established in the Mexican norms for the pouring of waste waters (CONAGUA-CEMARNA, 1997).

Huyard et al. (2000), in a two-stage system (thermophilic-mesophilic), achieved a reduction of fecal coliforms and helminth eggs in 5,5 and 2,6 decimal units in the reactor, with hydraulic retention time (HRT) of 2 days. Similar results were also reported by Gantzer et al. (2001) and Cabirol et al. (2002).

On the other hand, when studying Upflow Anaerobic Sludge Blanket (UASB) reactors, with capacity between 3 500 and 28 000 m³, Luu et al. (2014) found concentrations of fecal coliforms of 9,2 x 10⁶ MPN upon the entrance; while in the output effluents they oscillated between 1,1 x 10⁵ and 2,2 x 10⁶ MPN/100 mL, for a reduction of barely a decimal unit, much lower than the ones obtained in the covered lagoons of this study.

These results indicate that the completely agitated systems, with hydraulic retention times of more than 10 days, can reach an efficiency of pathogen reduction higher than the one reported in UASB reactors; because in these last ones, as they show dead zones and short hydraulic circuits, the lack of mixing causes a low efficiency in the elimination of pathogens (Terreros-Mecalco et al., 2009).

Regarding the parasite agents, in P-3 at the end of the anaerobic process they had been reduced from 58 to 26 e/L, and a total elimination was achieved at the end of the compensation lagoon. T-2.1 showed a similar performance, because the eggs decreased from 39 to 21 e/L in the digester and were totally eliminated in the stabilization lagoon.

This coincides with the report by Luu et al. (2014), who achieved a total elimination of the parasite eggs when using several digester technologies. According to these authors, the decrease of the infective elements is due, probably to the size and shape of the eggs, which are retained in the sludge bed.

The covered lagoon biodigester system also showed better results than the ones referred to in literature for large-size digesters, where only removals of 89,6 % were reached (Figueroa et al., 2004).

Seghezzo et al. (2002) achieved an elimination of eggs of only 86-97 % in a UASB reactor operated at 5,5 hours of HRT, from which it is inferred that for this indicator, the technology of covered lagoons shows better efficiency.

According to the results it is concluded that the covered lagoons had an adequate performance in the treatment of pig production resides and that this technology is feasible for being used in Cuba.

Received: April 15, 2015
Accepted: October 14, 2015