Upgrading and evaluation of a simple pond system for small communities with simple interventions to reduce land requirements and increase performance

In wastewater treatment, it is ideal to combine low area requirements, high efficiency and operational simplicity. Upgrading wastewater treatment systems can prove to be an expensive undertaking, especially for more resource dependent and complex systems such as activated sludge, biofilters and biodiscs. Natural treatment systems, like waste stabilisation ponds, may be also difficult to upgrade if this involves physical expansion of the system. However, a good approach is obtained if upgrading is carried out by implementing internal measures without increasing the number of ponds or the overall plant size.

Pond systems are considered very good for treating domestic sewage, presenting good organic matter removal and, in the case of maturation ponds, excellent pathogen removal. In the last years, in some countries, anaerobic and facultative ponds have been replaced by upflow anaerobic sludge blanket (UASB) reactors, resulting in land savings, good organic matter removal (65%–75%), less risk of bad smells, major operation control and less temperature dependence for organic matter removal (Chernicharo 2007). However, UASB reactors are not synonymous with high quality effluent, as Chernicharo (2007) and Oliveira & von Sperling (2011) pointed out, requiring additional units to remove reminiscent organic matter, nutrients and pathogenic organisms.

Maturation ponds following UASB act as a polishing step by removing pathogenic organisms and ammonia, while still promoting complementary organic matter removal (Dias et al. 2014). Shallower than anaerobic and facultative ponds, maturation ponds present a favourable environment (high concentrations of dissolved oxygen, high pH levels and good UV incidence across the whole water column) (Cavalcanti et al. 2001; Jungfer et al. 2007), therefore enhancing removal efficiencies (thermotolerant coliforms removal of 99.999%) (von Sperling 2008) and allowing for agricultural irrigation (Santos et al. 2009) following WHO (2006) guidelines. The quality of the final effluent usually depends on the number of ponds in series and the overall hydraulic retention time (HRT).

This paper aims at showing a simple and easy way to upgrade and improve an existing maturation pond treatment line, requiring only simple interventions over the previous system, originally comprised of a UASB reactor, three maturation ponds in series and a rock filter inserted in the final third of the last pond of the series. The previous setup had been in operation for over 10 years and was well documented in an overall performance evaluation paper by Dias et al. (2014). Other researches have been conducted on this previous system documenting different evaluations on specific subjects, and they can be found, for instance, in Araújo et al. (2010), Godinho et al. (2011), Possmoser-Nascimento et al. (2014) and Rodrigues et al. (2015). Flow-through or continuous flow ponds are used here, since they are the most common type of pond units, compared with the alternative of batch-fed ponds.

The new treatment line maintained the UASB reactor and the first maturation pond. The second maturation pond received longitudinal baffles and the pond depth was reduced to half of the previous setup. The third pond was entirely converted into a graded rock filter (GRoF) with decreasing grain size along the length (gravels #3, #2 and #1). The previous setup used 2.0 to 2.5 m².inhabitants⁻¹ to treat wastewater at a lower flow rate, while the new setup required only 1.5 m².inhabitant⁻¹ to treat 40 m³·d⁻¹, approximately 250 population equivalents. The various interventions allowed several studies to be conducted in order to endorse the proposed approach, such as hydrodynamic behaviour (shallow depth and low HRT) influence on the removal of specific constituents, the influence of the bottom sludge on the removal of specific parameters (with emphasis on nitrogen), and the potential of clogging in the coarse filter. Thus, the objective of the study was to optimise an existing setup of a continuous flow system by reducing the number of ponds in series required for disinfection by lowering the water level and applying baffles in the second pond. Further polishing of the effluent in terms of organic matter (algae) formed in the preceding ponds was sought out by incorporating a rock filter as the final treatment step, therefore resulting in high quality effluent in a total of less than seven days of treatment.

The upgrade and monitoring were conducted at the Centre for Research and Training on Sanitation (CePTS UFMG/COPASA), located near the municipality of Belo Horizonte, Brazil, in a dedicated area at the Arrudas Wastewater Treatment Plant (latitude 19°53′42″S and longitude 43°52′42″W). The climate in the region, according to the Köppen classification system, is Cwa – tropical altitude, with average annual temperature of 22.1 °C and precipitation of 1,540 mm/year.

The first pond accumulated a large amount of sludge over the previous 11 years (Possmoser-Nascimento et al. 2014) and was investigated regarding ammonia and nitrogen fractions removal (Rodrigues et al. 2015).

At the beginning of 2014 the second pond and third pond underwent complete desludging and their bottom and embankments were recompacted and lined. Two longitudinal baffles were inserted in the second pond, with the flexibility of modifying their length (currently taking 90% of the pond's length) and height.

Before upgrading the second pond with baffles, and after desludging, it's performance was compared to that of the first pond (with accumulated sludge over 11 years of operation). During this test, both ponds operated in parallel, with the same depth, for approximately six months, receiving the same flow.

The third pond was transformed into a GRoF, with the particularity of consisting of three different grain sizes, decreasing from the inlet towards the outlet.

The UASB reactor is 2.0 m in diameter and 4.5 m in height, resulting in 14.2 m³ and a HRT of 0.4 d (Figure 1(a)). The reactor has been in continuous operation for over 11 years. No problems have ever arisen in its operation, nor have strong odours been detected in the vicinity.

The first pond (Figure 1(b)) of the series had the same age of the UASB reactor, and consequently had accumulated sludge on the bottom (approximately 40% of the useful volume) (Possmoser-Nascimento et al. 2014). In the upgrading, the first pond maintained its depth at around 0.77 m, while the second pond (Figure 1(c)) was very shallow, with an average depth of 0.44 m (Table 1), half of that from the previous setup. The length-to-breadth ratio of the second pond before employing baffles was 5:1 and, after the intervention it increased to 43:1. The expectation with the baffles was to achieve near plug-flow conditions and to reduce short circuiting and dead zones, essential elements for the removal of pathogenic organisms.

The third pond was converted into a GRoF by dividing it into three zones in series (Figure 1(d)). The rock filter operates in a continuous horizontal subsurface-flow mode, with a liquid height of 0.5 m and a free board of 0.1 m. The objective of the filter is to remove particulate organic matter, mainly algae, inflowing from the second pond unit. This is done when the pond effluent passes through the void spaces in the submerged graded rock bed, causing algae to adhere and settle onto the media and undergo biological degradation (Crites et al. 2014). Each zone was 8.0 m in length and 5.25 m in breadth (bottom dimensions). The grain sequence was from the largest to the smallest grain, therefore possibly avoiding any premature clogging caused by organic matter inflowing from the second pond. The three zones used gravel #3 (25.0–50.0 mm), gravel #2 (19.0–25.0 mm) and gravel #1 (9.5–19.0 mm). A granulometric analysis characterised the gravel and the main indexes are as follows: (a) Gravel 3: d₁₀ = 27 mm; d₉₀ = 48 mm; uniformity coefficient d₆₀/d₁₀ = 1.37; void ratio (porosity, ε) = 0.48; (b) Gravel 2: d₁₀ = 14 mm; d₉₀ = 26 mm; uniformity coefficient = 1.55; ε = 0.45; (c) Gravel 1: d₁₀ = 9 mm; d₉₀ = 18 mm; uniformity coefficient = 1.50; ε = 0.43.

Samples were collected from the raw sewage and the effluent from each unit. However, this paper presents only data from final effluent (rock filter), since its major objective is to investigate the overall system performance. Further details of intermediate points can be found in Dias (2016). The monitoring period started in January 2014 and ended in April 2016, resulting in two years and three months of nearly continuous monitoring, with some exceptions due to public and school holidays. Monitoring was performed on a weekly basis and sample numbers (n) varied for each parameter from 18 to 75. Sampling took place in the morning, usually between 08:00 and 09:30 and analysis were performed according to Standard Methods for the Examination of Water and Wastewater (APHA-AWWA-WEF 2009). E. coli quantification was done by the chromogenic substrate (Colilert^®) technique, as recommended by Godinho et al. (2011).

Hydrodynamic studies were performed using tracer tests and continuous temperature measurements to understand horizontal and vertical profiling of the ponds. While the use of tracers provides important hydrodynamic parameters for characterising the ponds, vertical temperature measurement aimed at evaluating thermal stratification and vertical mixing occurrences.

For the tracer studies, the tracer used was common table salt (sodium chloride – NaCl) diluted in an aqueous solution (tap water). The tests were performed using the stimulus-response technique with pulse tracer injections at the inlet and electrical conductivity analyses at the outlet. Details can be found in Passos et al. (2016a).

The insertion of the filter material in the third pond was to assist in the retention of algae generated in the previous ponds, however, as in any filter, continuous operation can potentially result in pore obstruction and clogging. Because the GRoF has been in operation for a short time, its evaluation could enhance the understanding of the colour patterns and the evolution of clogging found in treatment filters with longer operational times, as suggested by Cooper et al. (2008). A geophysical method, called ground penetrating radar (GPR) was applied by Matos et al. (2016), which emits electromagnetic waves which are reflected by the media (different dielectric properties) at different velocities. The differences caused by the distinct dielectric properties create different images of the media (Aranha et al. 2002). More details on this method can be found in Matos (2015) and Matos et al. (2016).

Table 2 shows a comparison between the final effluent quality of the previous and the upgraded setups. A Wilcoxon-Mann-Whitney U test with a confidence level of 95% was performed to compare both final effluent concentrations.

The major comment on the results shown in Table 2 is the overall very good quality obtained in the final effluent for most parameters of interest in domestic sewage treatment (physical-chemical and bacteriological), especially considering that a simple natural treatment system is applied. The effluent concentrations from the new treatment line are for the most part lower than the concentrations from the previous treatment line studied by Dias et al. (2014), with emphasis on suspended solids and organic matter, which are constituents that are generally given priority in sewage treatment in developing countries. In this regard, the coarse filter had a very important role. Two constituents (ammonia-N and E. coli) were already expected to have higher effluent concentrations in the new set up, because of the exclusion of the third maturation pond and substitution by the coarse filter. However, this was a compromise, in order not to increase land requirements and to widen up the scope of the treatment system in terms of a broader capacity of the major wastewater constituents. This is a choice that must be taken when deciding the desired quality for the final effluent. If the main objective of the final effluent depends on having an even better bacteriological quality and lower ammonia levels, then the incorporation of a third shallow pond should be considered; if a more balanced effluent is desired, with also low organic matter and suspended solids in order to comply with discharge standards, then a GRoF should be applied instead of the third shallow pond. When analysing that six out of nine constituents had better effluent concentrations in the new setup, its implementation is justified, especially taking into account that the overall effluent quality was very good. Even though bacteriological concentration was higher, the final effluent still complied with some types of restricted and unrestricted irrigation (<10⁴ MPN/100 mL, WHO 2006). However, if an even better quality in terms of coliforms is desired, than another pond could be inserted in the treatment line, after pond 2 and before the coarse filter.

Overall, there were daily periodic events of thermal stratification followed by vertical mixing (destratification) in both ponds. Therefore, the depth reduction was still associated with stratification, which is reported to occur in ponds with depths greater than five metres (Addy & Green 1997). This could be explained by turbidity that blocks part of the solar radiation that enters the pond, even in shallow ponds and especially during the summer months (Ukpong et al. 2006). Vertical temperature gradients were predominantly in the range of 0–7 °C/m. The ponds remained 56% of the time under thermal stratification and 44% in vertical mixing (Passos et al. 2016b). It is believed that vertical mixing enhances coliform removal, since it transports bacteria from the lower levels to close to the surface, where the inactivation mechanisms related to solar radiation are at their maximum.

It is remarkable to see that oscillations in the curves occurred at daily cycles and are attributed to thermal stratification events, causing movement of part of the tracer solution to the bottom, and then destratifying with vertical mixing. This is supported by the thermal profiles shown in Figure 2. In addition to the inherent hydrodynamic behaviour of the pond, the high dispersion number in the ponds may have also been a result of thermal stratification, regardless of their geometric configuration.

Various conversion and reducing mechanisms may have occurred at the same time and the predominance of one over the other depends on environmental conditions, pond hydrodynamics and the presence of bacteria responsible for the process. Generally, in tropical regions, substantial nitrogen removal occurs through sedimentation and algal assimilation, however, nitrification/denitrification is a simultaneous absorption process (Camargo-Valero et al. 2010; Mayo 2013). Temperature favours both bacteria and algae (photosynthesis), but the preferred substrate is the same (ammonia) for both organisms. This may decrease nitrification and consequently denitrification. However, there is a symbiotic relationship between algae and nitrifying bacteria in function of liquid oxygenation and CO₂ production, respectively. As these ponds present daily mixing cycles, oxygen is taken to the bottom, allowing for aeration of the liquid-sludge interface layer (Keefala et al. 2011). This feature promotes, in addition to oxygenation, nutrient input and an increase in temperature, which are important factors for bacterial activity favouring nitrogen removal. The maturation ponds behaviour suggests that nitrogen removal occurred probably through algal assimilation in the liquid column and sludge contributed to the removal through nitrification/denitrification and/or anammox activity.

Even though NH₃ desorption can be a mechanism for ammonia removal in maturation ponds, it is not included in the discussion here because previous studies undertaken in the same two ponds of the previous setup indicated that this factor accounted for only a minor fraction of the overall removal. Volatilised ammonia was captured by a chamber on the surface of the ponds, and the mass balance of ammonia nitrogen of the ponds showed that volatilisation represented only about 2% of the total removal of nitrogen from the polishing ponds (Assunção & von Sperling 2012). This is in line with findings from other authors, who have questioned the importance of ammonia volatilisation in the nitrogen balance in ponds (Camargo-Valero & Mara 2007, 2010).

As inferred by the GPR measurements, porosity in the transects of the GRoF (first and second zones) varied on a narrow range, from 0.44 to 0.46 m³·m⁻³. Even though the GRoF had started its operation recently, there would be a loss of porosity that could be explained by the heterogeneity of the filter medium and distribution of wastewater, besides dust from the rocks present in the pores and solids retained from wastewater (Chazarenc et al. 2003).

The GRoF may serve as a template for understanding images generated by the GPR, therefore allowing to monitor the progression of porosity losses over time, using, if necessary, remediation techniques in an early stage of operation in order to increase the probability of successively cleaning the bed. It also may enable understanding which factors predominantly cause clogging. For example, the applied organic load in the GRoF (154 kg BOD ha^−1·d⁻¹) is higher than those usually applied in horizontal subsurface flow constructed wetlands, which are systems widely studied regarding their clogging tendency. This could indicate that the GRoF could have earlier and more severe obstructions in the pores in accordance with authors like Zhao et al. (2009). On the other hand, the filter medium used in the GRoF is more wear and tear resistant, which for Matos (2015), is a major contributing factor to clogging.

• Reducing the area required and the total HRT for treating domestic wastewater impacted positively on the overall performance of the pond system. Several of the variables analysed were improved compared with the previous setup composed of three ponds in series. This was mainly due to lowering the liquid depth and the incorporation of the baffles in the second pond and the inclusion of GRoF as the final step in the treatment line. The improvements were mainly in suspended solids and organic matter removal. Two variables (E. coli and ammonia-N) did not improve their concentration in the final effluent due to the absence of a third pond, which would further promote the removal of these variables. Even though bacteriological counts were higher, the final concentration for E. coli is compatible with practices of restricted and unrestricted irrigation (>10⁴ MPN/100 mL).

• The hydrodynamic studies indicated that there were daily periodic events of stratified conditions followed by mixed conditions (destratification) in both ponds.

• Tracer tests suggested a general tendency of completely mixed conditions with a high dispersion number, even in the baffled pond (higher dispersion number than in the first pond). This indicated that the baffles in the second pond did not approach the flow regime to the idealised plug flow, suggesting the occurrence of short-circuiting.

• Replacing the third pond with a GRoF revealed beneficial and increased removal efficiencies of various parameters (BOD, COD and TSS). This type of intervention is highly recommended when low concentrations of suspended solids and organic matter are required, while offering complementary pathogenic organism removal.

• Before the upgrading phase, a test was performed by comparing both ponds operating in parallel, with the same volume, and receiving the same flow rate, with little differences in the performance of the pond with accumulated sludge and the pond with no sludge. In the pond with sludge, even with a lower HRT in the liquid layer, the sludge contributed to the removal process of nitrogen due to the presence of microorganisms from the nitrogen cycle and by creating a stratified environment in terms of oxygen concentration.

• The results using the GPR in the rock filter allowed estimation of the medium porosity, and may prove to be useful in the future for the assessment of the possible evolution of the clogging stage.

The authors would like to thank CNPq, Capes, Fapemig, Finep, COPASA, the Institute of GeoSciences at UFMG, the laboratory at UFV and the students who participated in sampling and analyses. This research was part of an international programme financed by the Bill and Melinda Gates Foundation for the project ‘Stimulating local innovation on sanitation for the urban poor in Sub-Saharan Africa and South-East Asia – SaniUp’ under the coordination of UNESCO-IHE, Institute for Water Education, Delft, Netherlands.