Abstract
The present research addresses the performance deficiencies of high altitude (~2,600 m amsl) Waste Stabilisation Pond (WSP) systems located in the upper Lerma river basin in the State of Mexico, in Mexico. These treatment plants form part of a network built as part of the ‘Integral recovery program of the Lerma River’, which was adopted in 1989 as a strategy to reduce the pollution of the Lerma river and increase the water use efficiency in the basin. However, the above systems which, with one exception, are built to a common 3-stage design comprised of single or parallel biodigesters, anaerobic and facultative ponds have presented operational difficulties since commissioned in the early 1990s. In particular, the system of Mexicaltzingo had been selected as a model for rehabilitation and extension with maturation ponds, after an extensive diagnostic surveillance performed in a prior project, had shown that under-performance was caused by under-design, hydraulic short-circuiting, adverse environmental conditions and poor operation and maintenance. The main target was to achieve the WHO (1989) and Mexican unrestricted reuse effluent standard (NOM-001-SEMARNAT-1996) of <1,000 Faecal Coliforms (FC)/100 ml and <1 Helminth ova/1. However, due to opposition to the project by a politically motivated minority and civil disturbances in the municipality, the project was relocated (with almost a 6 month delay) to a similar WSP system at Santa Maria Rayon with a community willing to cooperate. System extension, with maturation pond construction, was started in October 2001 simultaneously with a rehabilitation of the original paired, parallel biodigesters, anaerobic and facultative ponds by the local Water Authority (CAEM). For the extension an existing fish pond adjacent to the WSP was incorporated into 3 new ponds of similar geometry and dimensions. The first 2 were designated as maturation ponds, whereas the third one, was intended to be used as a combined fish and maturation pond. Slow progress in the rehabilitation works by CAEM resulted in an additional 5 month delay in the project. The extension work was split in two stages for research evaluation purposes: 1. Open maturation ponds designed upon the ‘classical’ Marais (1974) equation, and 2. conversion of the first two maturation ponds into channelled ponds to demonstrate that by increasing the L/W ratio hydraulic efficiency and therefore pathogen removal are enhanced. Using the HYDRO-3D computational fluid dynamic model calibrated with on-site wind data, pond bathymetry data and observed FC decay rates from batch studies, it was predicted that 5. channels per pond with a 30 cm gap were necessary to achieve a high effluent quality. It was also suggested that the channels should be built perpendicular to the prevailing wind direction to induce a helicoidal mixing, which was believed to benefit the performance. However, budget limitations only allowed for the construction of one 3 channel and one 5 channel pond using geomembrane for the divisions, thereby increasing the L/W ratio from 1:1 to 11:1 and 27:1 respectively. An intensive microbiological, physico-chemical and hydraulic evaluation was performed for all stages of treatment, and the performance of the two maturation pond extension stages were compared, to identify the parameters and design factors affecting pond efficiency. It was demonstrated that the facultative ponds were anaerobic due to overloading, and required an additional >60% area, which was effectively compensated for by the first maturation pond. It was found that high ammonia levels, growth of purple sulphur bacteria due to excess sulphate in the raw sewage, and often extreme daily temperature variations (0-27°C) with strong winds inducing short circuiting, were responsible for an unstable biochemistry and ecology in the following two ponds, resulting in poor performance with respect to FC removal. The overall FC removal performance achieved by the stage 1 three open ponds, with a combined Nominal Retention Time (NRT) of 34.2 days was 99.4% >1 log below that required to meet the effluent standard. For stage 2, although the first pond with 3 channels maintained its facultative conditions with no improvement of effluent quality, a major increase in pond performance was achieved in the following 5 channelled pond, despite persistent unstable conditions but with raised oxygen levels. The FC removal increased substantially as compared with the open stage 1, from 0.58 to 1.47 log. As the final non-channelled pond maintained its performance, it was confirmed that the improved retention time is a major determinand controlling the FC reduction. Despite a slightly reduced NRT of 33.1 days, in this stage, the overall FC removal increased to 99.89% with a final average concentration of 3,230 FC/100 ml, being only slightly above the standard. With the Helminth ova requiring a NRT >20 days, the standard was also achieved. As concerns reuse, it was noted that a physico-chemical characterisation was also vital to exclude any salinity hazard. Due to the high ammonia concentration the aim of cultivating fish had to be abandoned. The results obtained in stage 2 proved that the channel configuration under aerobic conditions produces a higher effluent quality due to an enhanced hydraulic flow towards plug flow. This was corroborated by tracer studies in the 5 channel pond, in which the hydraulic efficiency was >75%. The tracer study also demonstrated that in effect, and as predicted, under windy conditions the mixing effect produced a delay of the dye as compared with still air conditions. Furthermore, the small size of the gaps, in which turbulence is promoted, did not produce any evident short-circuiting, confirming the prediction by the CFD model. It is concluded that channels made of geomembrane with 30 cm gaps are a feasible and cost efficient means of improving effluent quality of maturation ponds, which can be used to increase the performance of existing ponds or reduce the construction area in new designs. The effects of wind can also be beneficial if carefully integrated in the layout.