Wastewater Stabilization Pond System Research Articles

Investigating the efficiency of Vertical Sub-Surface Flow Constructed Wetlands in the Reduction of Faecal Indicator Bacteria and Organic matter Under Varied Sizes of Gravel Substrate Aggregates

Constructed wetlands (CWs) polish wastewater prior to discharge into aquatic receptacles. Size variabilities of substrates used in CWS may have effects on the treatment efficiencies though there is scanty information regarding this aspect in VSSF CWs. To address this knowledge gap, a laboratory scale mesocosm experiment was set up to investigate the potential of a VSSF CW to reduce organic matter and FIB using various gravel substrate aggregate sizes. This consisted of three gravel size treatment units; <12.5, 12.5-18 and 18-24 mm in triplicates. 70 liters of pre-treated wastewater from the final wastewater stabilization pond (WSP) of Egerton University’s WSPs system was added to the units, allowed to settle for 6 weeks for development of biofilms, followed by periodic feeding of equal wastewater quantity on weekly basis and influent and effluent samples collected for 8 weeks for analysis. Results disclosed reduction efficiency of 95.2, 94.3 and 88.4 % for E coli in the fine, medium and coarse gravel aggregates respectively. Less than 20 % reduction efficiency was recorded for BOD5 in all gravel aggregate sizes. There was no significant variation on performance of the three gravel aggregate sizes in reduction of both FIB and BOD5 (p˃0.05). Poor performance in BOD5 reduction was related to absence of wetland macrophytes in the study. The relatively high reduction efficiency for FIB was attributed to other factors and processes such as predation, mechanical interactions, starvation, microbial interactions and natural die-offs. The study recommends assessing the combined effect of increasing the retention time, use of wetland macrophytes and incorporating various gravel aggregate sizes in order to increase the efficiency of VSSF CW in reduction of FIB and organic matter.

Assessment of sediment arsenic and iron occurrence and leaching potential in a potable water treatment plant wastewater stabilization pond system

Wastewater stabilization ponds (WSPs) are commonly used to reduce wastewater metal(loid) concentrations from drinking water treatment plants (DWTPs) through sedimentation. However, this results in increased sediment concentrations that can be released back into the overlying water. Thus, our goal was to evaluate the WSP metal(loid)s occurrence and leaching potential. Currently, a Saskatchewan based DWTP’s WSP system was investigated given historically elevated effluent As and Fe concentrations. The WSP consists of five ponds that were sampled on six occasions in 2019 and 2020. In addition, sediments were used in laboratory-based experiments to determine their leaching potential. Overall, the sediments were found to contain elevated concentrations of As and Fe of 25 to 400 mg/kg and 10 000 to 45 000 mg/kg, respectively. Leaching experiments indicated that the pond sediments could potentially release As and Fe with log Kd values ranging from 2.21 to 4.31 L/kg, whereas Fe ranged from 3.32 to 5.53 L/kg.

Assessment of a wastewater stabilization pond system for removal of arsenic, iron, and ammonia from reverse osmosis water treatment plant residual wastewater

Drinking water treatment plants (DWTPs) produce wastewaters with elevated concentrations of heavy metals, metalloids, ammonia, and other contaminants. These wastewaters require treatment via processes including wastewater stabilization ponds (WSPs). This study assessed the arsenic (As), iron (Fe), and ammonia (NH3) concentrations in a Saskatchewan DWTP WSP system of five sequential ponds. Drone imaging combined with flow and depth data was used to estimate retention times which increased from 7–9 to 8–10 days after the DWTP upgrade. Concentration trends showed Fe decreased from Pond 1 to 3 and increased in Ponds 3 and 5, while As decreased from Pond 1 to 5. Average effluent As concentrations of 10.6 µg/L were above the 5.0 µg/L guideline, while concentration guidelines for both Fe and NH3 were easily met post-upgrade in 2020. Several actions are recommended to ensure adequate WSP operation including dredging, aeration, and installing macrophytes capable of As uptake.

Ecological processes-driven distribution of net-algal diversity and carbon sequestration potential across the sewage effluent gradient of stabilization pond system

Spatial and seasonal variability of diversity, abundance and carbon sequestration potential of net-phytoplankton were examined across the sewage effluent of six wetlands in waste stabilization ponds used for the treatment of municipal wastewater cum fish culture. The values of the Shannon-Weiner diversity index for Myxophycaeae, Euglenophyceae, and Chrysophyceae were higher in the facultative pond compared to maturation ponds, whereas that of Bacillariophyceae was higher in later than in former. The gross and net primary productivity of phytoplankton ranged from 3.594gCm−2d−1 to 32.858gCm−2d−1 and 2.34Cm−2d−1 to 15.227Cm−2d−1, respectively. The P/R ratio varied from 1.365 to 5.729 in different ponds investigated. Two clear-cut zones were well recognized. The facultative pond with the supremacy of blue-green algae favoured immense carbon sequestration potential. The total phytocarbon biomass in the facultative pond (60,491mgCm−3) was 2.5–4 folds higher than in subsequent maturation ponds. The carbon sequestration potential of the entire wastewater stabilization pond system was calculated to be 133,279mgCm−3. Further research is needed to prepare carbon budget by quantifying the carbon capture from atmosphere and emission into the atmosphere, if any through mud-water interactions of the bottom sediment.

Effects of Environmental Factors on the Disinfection Performance of a Wastewater Stabilization Pond Operated in a Temperate Climate

Treatment in a wastewater stabilization pond (WSP) relies on natural purification processes, which can be sensitive to both location and climate. This study investigated the effects of three environmental factors, pH, dissolved oxygen (DO) and temperature, on disinfection efficiency in a WSP system consisting of three facultative cells, and operated in a temperate climate region, in Eastern Ontario, Canada. Indicator organism (Escherichia coli (E. coli)) removal in WSP systems is driven by a combination of different factors. Elevated pH and DO concentrations, which are attributed to the presence of algae, are important factors for effective disinfection. Therefore, the presence of algae in natural wastewater treatment systems can contribute appreciably to disinfection. Consequently, based on algal concentrations, removal efficiencies of pathogenic microorganisms during wastewater treatment over the course of a year can be highly variable, where higher removal efficiencies would be expected in summer and fall seasons.

Modelling antibiotics transport in a waste stabilization pond system in Tanzania

Antibiotics in wastewater have become a growing problem in urban and peri-urban areas in developing countries as a result of increased use and misuse of antibiotics. A simple dynamic model, that describes the most important removal processes of antibiotic from the wastewater stabilization pond system (WSP) “Mafisa” in Morogoro, Tanzania, was developed using STELLA® software package. The model was based on liquid chromatography tandem mass spectrometry (LCMS/MS) analysis of trimethoprim, in water collected in the WSP. Concentrations of trimethoprim measured in the dry season and the rainy season were used in development of the model. To determine the model's applicability to simulate the removal of trimethoprim, a calibration was performed using concentrations from the dry season and a validation was performed using concentrations from the rainy season. To test the model's capacity to simulate the removal of other antibiotics than trimethoprim, a second validation was performed for three other antibiotics; metronidazole, sulfamethoxazole and ciprofloxacin. A two-tailed t-test with a confidence interval of 95% showed no significant difference (P=0.7819) between the values given by the model (CSIM) and the values measured by LCMS/MS (COBS) of the first validation, and the standard deviation (SD) between the differences was 1%. The second validation gave a mean SD=18% (found by a two-tailed t-test with a confidence interval of 95%) of the differences between CSIM and COBS. The model was developed under the assumption that settling, biodegradation, hydrolysis and photolysis were the only removal processes other than outlet. The major removal processes for trimethoprim and sulfamethoxazole were through settling and the outlet. Ciprofloxacin was removed by settling in the first pond. Metronidazole was mainly removed through the outlet, but settling and hydrolysis/photolysis also played a role. A sensitivity analysis (±10%) showed that the soil adsorption coefficient, the amount of suspended matter and the ratio of flow rate and volume were the most sensitive parameters. To strengthen the model, the exact removal processes should be further analysed. To apply the model on other WSP, a calibration of the settling rate constant and the amount of suspended matter should be performed.

A survey on the ratio of effluent algal BOD concentration in primary and secondary facultative ponds to influent raw BOD concentration

It is well known that algae play an important role in treatment of municipal wastewater with facultative ponds. The objective of the present study was to determine the ratio of effluent algal BOD concentration in facultative ponds to influent BOD concentration. This descriptive analytical study was carried out on a full-scale wastewater stabilization pond system located in Islamabad Gharb, northeast Iran. A total of 138 grab samples were collected from Primary and Secondary Facultative Ponds (PFPs and SFPs) twice per month over a period of one year and analyzed for algal cell number, chlorophyll a, and BOD5 concentrations. The mean ratios of effluent algal concentration of PFP5 and SFP7 to influent BOD5 of PFP5 were 1.11±0.53 and 1.59 ± 0.66, respectively. Also, the mean ratios of effluent algal concentration of PFP6 and SFP8 to influent BOD5 of PFP6 were 1.20±0.63 and 1.57 ± 0.68, respectively. Except PFP6, significant differences were found in the means of the respective ratio in PFP5, SFP7, and SFP8 between cold and warm seasons (p < 0.05). Although there was a significant difference in the means of the respective ratio between primary and secondary facultative ponds (p < 0.05), there was no significant difference between the same ponds in modules (p > 0.05). Lower effluent algal concentration to influent BOD5 ratio found in cold months indicates strong dependency of algal photosynthetic activity to sunlight and temperature. Also, the effluent algal concentration in secondary facultative ponds was higher than that in primary facultative ponds, since better condition is provided by secondary facultative ponds for algal–bacterial activity.

Taenia eggs in a stabilization pond system with poor hydraulics: concern for human cysticercosis?

The objective of this study is to compare the removal of Taenia eggs to the removal of Ascaris eggs in a wastewater stabilization pond system consisting of three ponds in series, where the hydraulic residence time distribution has been characterized via a tracer study supported by computational fluid dynamics modeling. Despite a theoretical hydraulic retention time of 30 days, the peak dye concentration was measured in the effluent of the first pond after only 26 hours. The smaller-sized Taenia eggs were detected in higher concentrations than Ascaris eggs in the raw wastewater. Ascaris eggs were not detected in the pond system effluent, but 45 Taenia eggs/L were detected in the system effluent. If some of these eggs were of the species Taenia solium, and if the treated wastewater were used for the irrigation of crops for human consumption, farmers and consumers could potentially be at risk for neurocysticercosis. Thus, limits for Taenia eggs in irrigation water should be established, and precautions should be taken in regions where pig taeniasis is endemic. The results of this study indicate that the theoretical hydraulic retention time (volume/flow) of a pond is not always a good surrogate for helminth egg removal.

Distribution and spatial variability of sludges in a wastewater stabilization pond system without desludging for a long period of time

This paper describes the sludge accumulation and its characteristics after several years of operation (15–20 years) of an urban wastewater treatment system using stabilization ponds (anaerobic, facultative, and maturation ponds) in southern Spain. The rate of sludge accumulation (0.011m3/person year for anaerobic pond) was lower than previously reported by other authors (0.04m3/person year) due to the effects of degradation and consolidation after this long period of time, and the vertical distributions of humidity and volatile solids indicate a consolidation and mineralization of sludge with depth. Confirmation of this fact by new experimental data might require a reconsideration of desludging times in such systems.Principal component analysis revealed some specific features of the data structure, and three principal components were identified which collectively accounted for 91.1% of the total variance. Principal components analysis results were confirmed by cluster analysis. Three clusters of variables were detected, corresponding to the three previously identified components. These results confirm that there are clear differences in the physico-chemical properties of sludges deposited in each pond. Vertical distributions of parameters indicate a consolidation and mineralization of sludge with depth. This supports the hypothesis of consolidation, mineralization, and volume reduction of sludge after long periods of time.

Nitrogen removal in wastewater stabilisation ponds

The objective of this work was to evaluate the nitrogen removal in a wastewater stabilisation pond (WSP) system and to assess the relative contributions of ammonia volatilisation and nitrificationdenitrification to the overall N-removal. The water quality at the inlet and outlet of each treatment stage of the Meze-France WSP system was monitored fortnightly, from July 2003 to August 2004. The mean nitrogen removal was 65%. Nitrogen removal was higher in summer (89%) than in winter (38%). Maturation ponds provided the bulk of the nitrogen removal. The water quality was monitored from April 1st to July 13th 2005 at the inlet and outlet of the third 2 ha maturation pond. Volatilization has been assessed through literature data complemented by specific experiments. Ammonia volatilisation was low. Nitrification took place during warm periods. In June, oxidised nitrogen content reached 30 mg L-1, then decreased when denitrification occurred. Nitrification-denitrification seems to be the major pathway for nitrog...

Feasibility of a constructed wetland and wastewater stabilisation pond system as a sewage reclamation system for agricultural reuse in a decentralised rural area

The performance of a constructed wetland (CW) and wastewater stabilisation pond (WSP) system for sewage reclamation and paddy rice irrigation in a decentralised rural area was examined using a feasibility study. The CW was satisfactory for sewage treatment, with good removal efficiency even in the winter period, but the effluent concentration was relatively high in the winter period owing to the high influent concentration. The CW effluent was further treated in a WSP and the WSP effluent was considered safe for crop irrigation with respect to sewage-borne pathogens. Reclaimed water irrigation did not adversely affect the yield of rice; on the contrary, it resulted in an approximately 50% greater yield than in controls. The chemical characteristics of the soil did not change significantly during the experimental period of irrigation with reclaimed water. In the winter, CW effluent could be stored and treated in a WSP until the spring; the water could then be discharged or reused for supplemental irrigation during the typical Korean spring drought. Overall, sewage treatment and agronomic reuse using a CW-WSP system could be a practical integrated sewage management measure for protecting receiving water bodies and overcoming water shortages in decentralised rural areas.

Performance of a small wastewater stabilisation pond system in tropical climate in Dar es Salaam, Tanzania

Waste stabilisation ponds (WSP) are widely used in Tanzania. Their traditional design focuses on BOD and pathogen removal, but nutrient removal is equally important. WSP performance was evaluated to come up with information that would be used to evaluate pond performance. Samples were analysed twice a month for six months. Results showed total and faecal coliform removal by 4 log units (99.96 and 99.98% respectively). There was partial reduction of COD, BOD, (46% and 27% respectively), conductivity and total dissolved solids (32.6 and 32.4% respectively). Variation for the nutrients like inorganic phosphorus, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen was not consistent and there was practically no reduction, although there is some nitrification taking place. From this study it may be concluded that BOD and nutrients may not be useful to evaluate pond performance. Instead parameters such as conductivity, total dissolved solids, coliform bacteria, dissolved oxygen, pH and chlorophyll would be suitable for that purpose, due to their consistent variation within cells of the pond system. But BOD and nutrient removal are important and have to be improved to enhance treatment in the WSP.

Removal of faecal coliforms in a wastewater stabilisation pond system in Mindelo, Cape Verde

In Mindelo, Cape Verde, domestic wastewater is treated in a waste water stabilisation pond system with the aim of reusing the water for agricultural purposes. The conductivity in influent wastewater is high and in order to minimise an increase in the conductivity the plant is operated with as low retention time as possible. An investigation was performed over two and a half months in 1995 with the aim of finding factors that affect the removal of faecal coliform bacteria. COD, conductivity, flow, pH and faecal coliforms were then followed through the system. The results show that the die-off rate, K, is high when the pH is high. The results also indicate that the retention time seems to be the most important factor, which means that the plant has to be operated with at least 3-4 maturation ponds in series and the inlets and outlets have to be constructed in a way that avoids short-circuiting. It was also found that not only the evapuation but also chemical and/or biological reactions affect the conductivity. This phenomenon will be investigated further.

Low-cost upgrading of an over-sized waste water stabilisation pond system in Mindelo, Cape Verde

Water is scarce in Cape Verde. In the city Mindelo almost all fresh water has to be produced by desalination of sea water. In this perspective, reuse of treated waste water for agricultural purposes is a very interesting alternative. The waste water stabilisation pond system in Mindelo is over-sized, with high evaporation and increasing salinity in the effluent water as a result. The salinity is already very high in the influent and with present process configuration the effluent is unusable for irrigation. The treatment plant has to be operated with less surface area to minimise the evaporation. This, on the other hand, means that an acceptable bacteriological quality in the effluent will not be achieved. By subdividing the ponds with removable plastic tarpaulins, a much smaller volume is needed to achieve this. The plastic tarpaulin can be moved and the process may be configured to different influent flows. The technique of subdividing ponds with a plastic tarpaulin is achieved at low cost and is very simple. The bacterial die-off constant, K, in the maturation ponds has been found to be around 2.65 d−1 at 26°C, which is comparable to results determined elsewhere. The bacterial die-off constant in the anaerobic pond was found to be much lower, 0.3 d−1. Operational strategies for different flows are presented.

Low-cost upgrading of an oversized waste water stabilisation pond system in Mindelo, Cape Verde

Water is scarce in Cape Verde. In the city Mindelo almost all fresh water has to be produced by desalination of sea water. In this perspective, reuse of treated waste water for agricultural purposes is a very interesting alternative. The waste water stabilisation pond system in Mindelo is over-sized, with high evaporation and increasing salinity in the effluent water as a result. The salinity is already very high in the influent and with present process configuration the effluent is unusable for irrigation. The treatment plant has to be operated with less surface area to minimise the evaporation. This, on the other hand, means that an acceptable bacteriological quality in the effluent will not be achieved. By subdividing the ponds with removable plastic tarpaulins, a much smaller volume is needed to achieve this. The plastic tarpaulin can be moved and the process may be configured to different influent flows. The technique of subdividing ponds with a plastic tarpaulin is achieved at low cost and is very simple. The bacterial die-off constant, K, in the maturation ponds has been found to be around 2.65 d-1 at 26°C, which is comparable to results determined elsewhere. The bacterial die-off constant in the anaerobic pond was found to be much lower, 0.3 d-1. Operational strategies for different flows are presented.

Removal of faecal coliforms in a wastewater stabilisation pond system in Mindelo, Cape Verde

In Mindelo, Cape Verde, domestic wastewater is treated in a waste water stabilisation pond system with the aim of reusing the water for agricultural purposes. The conductivity in influent wastewater is high and in order to minimise an increase in the conductivity the plant is operated with as low retention time as possible. An investigation was performed over two and a half months in 1995 with the aim of finding factors that affect the removal of faecal coliform bacteria. COD, conductivity, flow, pH and faecal coliforms were then followed through the system. The results show that the die-off rate, K, is high when the pH is high. The results also indicate that the retention time seems to be the most important factor, which means that the plant has to be operated with at least 3–4 maturation ponds in series and the inlets and outlets have to be constructed in a way that avoids short-circuiting. It was also found that not only the evaporation but also chemical and/or biological reactions affect the conductivity. This phenomenon will be investigated further.

Graphical presentation of the transformation of some nutrients in a wastewater stabilization pond system

In order to evaluate the transformation processes of carbon, nitrogen and phosphorus in a wastewater stabilization pond system visually and quantitatively, material transformation graphics of these three elements are produced from the ecological model of a pond system. The model was established by the authors based on their own experimental experiences. The results of the analyses of these graphics indicate that the removal rates of carbon are closely related to its transformation quantities; the mechanisms of the wastewater stabilization pond and its difference from the traditional biological treatment system are interpreted further in this study. The study shows that the major characteristics of a pond system beyond the traditional biological treatment system are that the pond system possesses a longer food chain which makes the system more stable and maintains a perfect relationship between bacteria and algae which makes good oxygen and nutrient cycling. Removal of nitrogen and phosphorus mainly depends on the nutrient assimilation of bacteria and algae, since microbial biomass is limited in the pond system, very high removal rates of nitrogen and phosphorus should not be expected using a stabilization pond.

Modelling Coliform Reduction in Wastewater Stabilization Ponds

The paper addresses faecal coliform reduction in a large wastewater stabilization pond system. It was found that values of the faecal coliform reduction rate coefficient (Kb) increased with increasing water temperature (Tw), solar radiation (Rs) and pH and with decreasing total BOD5 (CBOD), soluble BOD5 (SBOD) and surface organic loading (BA,CBOD) Correlation analysis has shown that Kb is more responsive to Tw than to Rs and to SBOD than to CBOD. An optimized-parameter model was developed to describe Kb in secondary ponds.

Modelling Nematode Egg Elimination in Wastewater Stabilization Ponds

The paper analyses data on intestinal nematode egg numbers (NNE) taken over a 30-month period in a large wastewater stabilization pond system. The retention time of 8-10 days suggested by WHO as the treatment expected to achieve an effluent with less than 1 nematode egg per litre was not supported by the results. A model has been developed to describe the elimination of nematode eggs(NNE) in ponds. According to this model, 14 days theoretical retention time is required to achieve the above criterion in the Alsamra pond system. The importance of the inflow jet velocity in influencing effluent quality, in terms of NNE, has been demonstrated.

Sedimentation and Digestion on Pond Bottoms – An Attempt to Establish a Short-Term Material Balance

Sedimentation and digestion have been measured in situ on the bottom of the first (facultative) and the last (maturation) pond of a wastewater stabilization pond system during six periods of one week. Calculated dry solids balances of the ponds bottoms showed that most of the collected solids would have an endogenous origin. Identification of the collected solids by digestion in the laboratory and supplemental in situ measurements in winter, when gas production was low, allowed the conclusion that the collected solids actually were fairly stabilized bottom sediments which are resuspended by the vigorous gas production. The establishment of a short term sedimentation-digestion balance appears to be a practical impossibility on an actively fermenting pond bottom.