The wastewater treatment plant (WWTP) of the Empresa Municipal de Agua Potable y Alcantarillado (EMAPET) is located in Barrio Vista Hermosa, San Benito, Petén, Guatemala. The WWTP from EMAPET oversees the wastewater treatment in the Central Area located in San Benito, Petén. The tributary of the WWTP has mainly domestic origin, however, it has some tributaries of commercial activities that are important due to their special nature.
The EMAPET WWTP has a total of 12 ponds (Fig. 1) and a constructed wetland planted with Eichhornia crassipes which is operating since 2019. The first process corresponds to the primary treatment and is made up of four anaerobic ponds (Fig. 2), the secondary treatment is composed by four facultative lagoons and the tertiary treatment is composed by four maturation ponds. In anaerobic lagoons microorganisms are used to decompose organic matter in an anoxic medium in order to remove between 50 to 60% of the BOD. Anaerobic lagoons have a retention time of approximately 5 days and depths of between 3 to 5 m.
In facultative lagoons (Fig. 3) it is carried out a secondary treatment, where aerobic and anaerobic microorganisms decrease pollutants by stabilizing organic matter. The oxygen necessary in this stage enters the lagoon through the fall of an inlet structure and by the movement of the water itself caused by the flow that enters, in addition to the swell caused by the surface wind. The depth of these lagoons should be less, between 1.5 to 2 m and with a greater surface area. The retention time in this stage is 5 d and the BOD reduction is achieved by up to 90%.
Finally, in the maturation ponds (Fig. 4), tertiary treatment is carried out where the ponds are shallow (1.0 to 1.5 m) and have a large surface area. In this stage, the number of fecal coliforms is reduced, however, nutrients such as phosphorus and nitrogen are not completely removed, which are the main causes of eutrophication in water bodies. The retention time per pond is greater than two days.
In 2019, as part of a study of wastewater treatment to remove total nitrogen and total phosphorus, an additional wetland was constructed planted with Eichhornia crassipes (Fig. 5). This macrophyte is known as Water Hyacinth or Nymph.
The importance of a biological treatment with Eichhornia crassipes is due to the fact that nutrients such as nitrogen and phosphorus are not removed in the treatment lagoons of the EMAPET WWTP, which contributes to the increase in eutrophication levels in Lake Petén Itzá. High levels of nitrogen and phosphorus concentration cause the proliferation of certain algae that use these nutrients for their reproduction. The planted wetland has 2.5 m wide by 6 m long and 1 m deep. About 40 cm of gravel were placed as a substrate and black canvas as waterproofing to prevent the infiltration of sewage. The average flow is 1.5 m3/d with a retention time of 5 d. The experimental wetland was covered with approximately 150 to 200 plants and in addition to this, a grid was placed in the water outlet tube to prevent the plant material from going towards the effluent that empties into the Xucupó stream and in this way avoid the propagation of the Water Hyacinth in the stream.
The artificial wetland built to carry out the experiment corresponds to a surface flow type. The use of macrophytes has been used as an alternative secondary or tertiary treatment for wastewater treatment, proving to be an economical and efficient method in the removal of nutrients such as phosphorus and nitrogen.
Project Size: 9 ha (water table); 15 ha (the whole complex)
Wetland Type (System/Cells):
Free-water system; recircilation of treated stormwater; the plot for snow and ice removed from the city of Enköping
Company/Organization: Enköping Kommun (Municipality of Enköping)