Waste stabilisation pond system has been used more especially in developing countries for sewage treatment. The objective of this study was to investigate the hydraulic and performance efficiency of Palapye waste stabilisation ponds. The hydraulic efficiency was evaluated through drogue, pond geometry and sludge accumulation assessment. The performance efficiency was evaluated through periodic sampling and analysis of physiochemical and bacteriological parameters of individual units and of the system as a whole. Except for the maturation ponds, the depth of the anaerobic pond had reduced from 4 m to 0.45 m, for facultative ponds from 2.0 m to a range of 0.52 m - 0.91 m. The design hydraulic retention time of the system had reduced from 20 days to 7.1 days. The concentration of some physiochemical parameters in the effluent was 305 mg·L -1, 277 Nephelometric Turbidity Units (NTU), 204 mg·L -1, 156 mg·L -1, 110 mg·L -1, and 15 mg·L -1 being total suspended solids, turbidity, nitrates, chemical oxygen demand, biochemical oxygen demand and phosphate respectively. These values were more than the standard limits of the country. Effluent total coliforms concentration was 3.6 log units and within the threshold of 4.3 log units, faecal coliforms concentration was 3.5 log units, slightly higher than the threshold of 3 log units. Though Escherichia coli have no limits for discharge into other environments, the concentration in the effluent was reasonable at 2.5 log units and also within irrigation limit of 3 log counts. Palapye wastewater treatment system hydraulic efficiency is lower than the design criterion. The system was overall poor in physiochemical parameters removal but better in bacteriological removal.
The growing investment in water supply and reticulation in Botswana has resulted in many households’ connections and large quantities of wastewater generated which needs adequate treatment to meet national standards before disposal into the receiving environment. Treatment to required standards has been a serious challenge especially in developing countries where technology and skilled personnel has been found wanting. In developing world, waste stabilisation pond (WSP) has been widely used especially where land is abundant and climate is favourable especially temperature and sunlight [
Most previous studies have not included pond geometry, sludge accumulation and drogue to analysis hydraulic efficiency. In addition there are few or no studies that have incorporated the performances of individual units in the treatment system but included only the overall performances.
Over the past years, Botswana has experienced an increase in water demand due to the rapidly growing population. Household water connections generate wastewater that should be discharged safely into the environment. This has led to wastewater treatment facilities being constructed in the country. The treatment systems in the country are mostly waste stabilisation ponds. Palapye WSP system was commissioned in 1997 for treating sewage from domestic and industrial wastewater. Treated effluent is discharged into the catchment area of tributaries that drain into Lotsane River which is dammed downstream.
The aim of this study was to investigate the performance efficiency of the Palapye wastewater treatment plant efficiency. The associated objective was to investigate the hydraulic efficiency and performance efficiency of Palapye waste stabilisation ponds.
Palapye is situated almost halfway between Francistown and Gaborone in Central District at elevation 919 m in Botswana. Its geographic location is latitude −22˚33'00'' and longitude 27˚08'00''. The 2011 housing and population census was 36, 211. The Palapye wastewater treatment system is comprised of 1 anaerobic pond, 3 facultative ponds and 6 maturation ponds (
Grab samples were collected from both inlet and outlet points of the system or for each pond. Samples were placed into 1 L of either polyethylene or glass bottles depending on the parameters to be analysed. The bottles were first cleaned and rinsed with Deionised (DI) water and then with dilute hydrochloric acid and preserved as recommended by American Public Health Association (APHA) 1998 Standard Methods for the Examination of Water and Wastewater. Samples were preserved in ice box during transportation before analysis.
Some physiochemical and microbiological parameters (E. coli, Total Coliforms, and Fecal Coliforms) were analysed according to Standard Methods (APHA,
1998). Some of the samples were either sent to Department of Water Affairs in Gaborone or Betach (PTY) LTD Laboratories in Francistown for quality cheque. Temperature, pH, TDS, and EC were analysed at site using portable multiparameter TestrTM 35 series meter supplied by Thermo Fisher Scientific. Turbidity was analysed using DR 900 multiparameter portable colorimeter, supplied by HACH, United States of America. Sampling bottles were washed and sterilized in the autoclave. Heat tape was used for verification of equipment for sterilisation. Faecal coliforms (FC) were enumerated by the Membrane Filtration (MF) technique, as per APHA (1998) and the results expressed in Colony Forming Units (CFU) per 100 mL. Enumeration was done in Membrane Lactose Glucuronide Agar (MLGA). Inoculated plates were incubated at 44˚C ± 0.5˚C for 24 hours. All benches where bacteriological testing was conducted were cleaned and disinfected with 70% ethanol solution. A 100 mL sample was filtered through 0.45 μm membrane filter paper. The filter paper was placed on a specific media designed to allow colonies of the indicator organisms to grow. After an incubation period the colonies were counted and the results recorded.
The percent removal efficiency was calculated using the following formula:
% Removal = C i − C e C i × 100 % (1)
where: Ci is the concentration of the influent.
Ce is the concentration of the effluent.
The following parameters were investigated to assess the hydraulic performance of the system.
The design retention time was obtained from the detailed design by the Design Engineer. The effective hydraulic retention time was based on pond dimensions, that is, length, width and effective depth. The effective depth was determined by subtracting sludge blanket depth from design depth. The average flow for the system was calculated from inflow rates recorded over time. In determining actual retention time, a drogue method was used to assess the actual hydraulic retention time (HRT) of different units. Five runs were conducted for each pond between October 2016 and February 2017. A replicate of five oranges per run were placed at the inlet of each pond. Time taken by each orange to travel from inlet point to outlet position was recorded and the average time calculated thereafter. Drogue locations were noted at varying intervals, depending on the velocity of their travel. A number of attempts had to be abandoned due to poor weather conditions and scum on the pond surface that interfered with the movement of the floats.
The physical characteristics including the shape, volume, inlet to outlet alignment, depth of each pond were determined through field survey data. Length to width ratio, inlet and outlet depth was determined using measuring tape and calibrated ruler. Some measurements were taken and average values calculated. Pond shape, inlet and outlet positions were established by visual observations. The pond dimensions were determined by measuring the length and width of each pond.
Sludge depth in each pond was measured on three occasions, between June and August 2017, using the white towel method described by [
Graphs were prepared using Microsoft Excel 2010. Statistical analysis such as mean and standard deviations were also calculated using Microsoft Excel 2010.
Ponds | A1 | F1 | F2 | F3 | M1 | M2 | M3 | M4 | M5 | M6 | Total (d) |
---|---|---|---|---|---|---|---|---|---|---|---|
Dimensions (m) | 42*42*4 | 42*33*2 | 42*33*2 | 42*33*2 | 92*84*1.5 | 92*84*1.5 | 84*48*1.5 | 84*48*1.5 | 84*48*1.5 | 84*48*1.5 | |
Design HRT (hr or d) | 22.8 hr | 22.8 hr | 22.8 hr | 22.8 hr | 2.72 d | 2.72 d | 2.72 d | 2.72 d | 2.72 d | 2.72 d | 20 |
Effective HRT (hr or d) | 2.64 hr | 2.39 hr | 3.96 hr | 4.2 hr | 1.6 d | 1.6 d | 0.84 d | 0.84 d | 0.84 d | 0.84 d | 7.1 |
Actual HRT (d) | 1.2 | 1.6 | 1.8 | 1.6 | 2.8 | 2.8 | 3 | 3.6 | 1.8 | 1.4 | 21.6 |
In
The length to width ratios were ranging between 1:1 to 1.3:1 and 1.1:1 to 1.75:1 for facultative and maturation ponds respectively (
Pond | A1 | F1 | F2 | F3 | M1 | M2 | M3 | M4 | M5 | M6 |
---|---|---|---|---|---|---|---|---|---|---|
L:W Ratio | 1:1 | 1.3:1 | 1.3:1 | 1.3:1 | 2:1 | 1.1:1 | 1.1:1 | 1.75:1 | 1.75:11 | 1.75:1 |
of flow in these ponds. As for maturation ponds, lengths to width ratio ranged between 1.1:1 and 2:1. Two of the ponds had a ratio of 1.1:1 with the remaining four being 2:1 or nearly, which was similar to the recommend values in most cases.
Higher L:W ratio shifts the pond towards plug flow conditions, which is associated with improved hydraulic performance [
Inlet and outlet positions for the system are shown in
Inlet and outlet depth varied from pond to pond. The recommended outlet depths by [
Pond | A1 | F1 | F2 | F3 | M1 | M2 | M3 | M4 | M5 | M6 |
---|---|---|---|---|---|---|---|---|---|---|
Inlet (mm) | 600 | 600 | 510 | 510 | 700 | 1200 | 1200 | 1200 | 700 | 600 |
Outlet (mm) | 600 | 600 | 510 | 700 | 115 | 1200 | 1200 | 700 | 600 | Outlet pipe |
F2 the rest of the units (A1, F1, F3 and all the maturation ponds) will not experience channelling of wastewater.
The sludge blanket heights in each pond are shown in
In this study, volume reductions were 90%, 78%, 76% and 60% respectively, for A1, F1, F2 and F3. Such large reductions were expected as the ponds have been in operation for over 20 years without desludging. The presence of sludge bench was noticeable near inlet and outlet points indicating the presence of sludge in the ponds therefore decreasing the effective HRT and overall performance of the system.
Ponds | A1 | F1 | F2 | F3 | M1 | M2 | M3 | M4 | M5 | M6 |
---|---|---|---|---|---|---|---|---|---|---|
Design depth (m) | 4 | 2 | 2 | 2 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 |
Actual depth (m) | 0.45 | 0.52 | 0.86 | 0.91 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 |
Sludge blanket depth (m) | 3.55 | 1.48 | 1.14 | 1.09 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
Design volume (m3) | 7056 | 2772 | 2772 | 2772 | 11562 | 11562 | 6048 | 6048 | 6048 | 6048 |
Effective volume (m3) | 705 | 623 | 670 | 1108 | 11562 | 11562 | 6048 | 6048 | 6048 | 6048 |
It was observed earlier that the effective HRT was lower compared to design HRT which could have led to less contact time for microbes to degrade BOD. BOD is mostly removed in anaerobic and facultative ponds but it was observed that design HRT for each of the first four ponds was 22.8 hours and the corresponding effective HRT for the same ranged between 2.39 hours to 4.2 hours. This was a higher reduction in residence time, hence channelling and short circuiting effect contributing to inefficiency. This was in agreement with [
On average, influent and effluent COD concentrations were 549 ± 145 mg∙L−l and 156 ± 135 mg∙L−l respectively representing a percent removal averaging 72%. The system did not meet the threshold of 150 mg∙L−l though this was high removal of COD. However these results are relatively less compared to other WSPs which can achieve more than 80% removal as reported by [
achieved by an appropriate retention time which was not the case in this study. COD removal has been reported to be due to physical and microbial mechanisms [
It was observed that TDS concentrations averaged 586 ± 104 mg∙L−l and 536 ± 82 mg∙L−l in the influent and effluent respectively, which was 8.5% reduction. Though this was very low removal efficiency, the standard limit of 2000 mg∙L−l was not exceeded. Poor removal was attributed partly to high overloading of the ponds and heavy accumulation of sludge in the primary sedimentation ponds which was the case in this study. High concentrations of TDS reduce water clarity, contributing to a decrease in photosynthesis hence low oxygen supply for biodegradation.
Average TSS concentrations in the influent and effluent were 294 ± 57 mg∙L−l and 305 ± 83 mg∙L−l respectively, which was an increase of 3.74%. TSS in wastewater can absorb heavy metals during treatment and introduce heavy metals into the receiving environment [
Nitrates had increased in the effluent from an initial average concentration of 1.58 ± 0.3 mg∙L−l to 203 ± 53 mg∙L−l in the effluent which was a 99% increase. The effluent concentration was way above the permissible limit of 50 mg∙L−l. The increment could have been due to algae die off which exerted nitrates since algae utilise nitrates. Such increase was reported by [
Similarly, it was observed that there was an increase of phosphate ion in the effluent as the average influent concentration was 12.4 ± 0.58 mg∙L−l compared to average effluent concentration of 15.0 ± 6.7 mg∙L−l, a 20% increase. This concentration exceeded the national threshold of discharge into other environments, of 1.5 mg∙L−l. It has been reported by [
The concentrations of chloride (Cl−), calcium (Mg2+) ,potassium (K+) and sodium (Na+) ions are as shown in
The mean inluent and effluent concentrations values for K+ were 27.9 ± 14 mg∙L−l and 28.3 ± 0.7 mg∙L−l respectively. This shows a very low removal of K+ and the values far exceed the limit discharge of 0 mg/l allowed into the surface water. Other ions such as Na+, Ca 2+, F− Mn2+ and SO 4 2 − were within the threshold limit and therefore not hazardous to the environment.
Electrical conductivity values were 1355 ± 45 µS/cm and 519 ± 65 µS/cm in the influent and effluent respectively, which was an average of 62% removal (
The average pH in the influent and effluent were 7.03 ± 0.5 and 6.87 ± 0.4 respectively (
Parameter | Inlet concentration | Outlet concentration |
---|---|---|
EC (µS/cm) | 1355 ± 45 | 519 ± 65 |
pH (units) | 7.03 ± 0.5 | 6.87 ± 0.4 |
Turbidity (NTU) | 415 ± 1 | 277 ± 2 |
Temperature (˚C) | 25.3 ± 3 | 25.3 ± 0.14 |
not acceptable because it affects survival of aquatic life. In addition, high pH interferes with the optimum operation of wastewater treatment system. At acidic conditions, heavy metals tend to exist as free metal ions while around neutral conditions, some precipitate as hydroxides or other insoluble species if the appropriate co-ion is available [
Average turbidity concentrations in the influent and effluent were 415 ± 1 NTU and 277 ± 2 NTU respectively which was 33.3% reduction. This was a low achievement as this value was above the threshold limit of 30 mg∙L−1. Observed algae growing in the ponds could have contributed to high turbid effluent. Poor hydraulic efficiency of the system could have contributed to poor settling of flocs and suspensions. High effluent TSS concentration observed earlier could also have contributed to high turbidity. Desludging of the ponds could increase HRT and improve the hydraulic efficiency of the system and overall performance on turbidity removal.
The average influent and effluent temperatures were 25.4 ± 3 and 25.3˚C ± 0.1˚C respectively. The maximum and minimum values (not shown) in the effluent were 33.3 ± 2.6 and 17.0˚C ± 1.6˚C. It has been reported that working temperature for anaerobic conditions is between 25˚C to 40˚C and that anaerobic conditions decrease rapidly when temperatures are below 15˚C [
The influent and effluent bacteriological results are shown in
Reference [
The performances of individual units on the reduction of BOD, TDS, COD and pH are shown in
the range of 6.5 to 9 units and discharges should not alter the ambient pH by more than 0.5 pH units in mixing zones [
The mean concentrations of TDS, BOD and COD in each operational unit indicate a reduction as wastewater passed through each pond. There is a noticeable pattern in TDS, BOD, and COD values obtained in A1 where there was higher concentration of these parameters than in the influent. This was expected as it is the first pond in the system and the wastewater had to spend some time for treatment. There was marginal decrease in TDS, BOD and COD concentrations in the system from F1 to M4 (
The same pattern was observed for COD, but there was no sharp rise as observed in the case of BOD. There were marginal fluctuations between the ponds, with reductions ranging between 22 mg∙L−l and 43 mg∙L−l and increases between 3 mg∙L−l and 4 mg∙L−l. COD removal in A1, facultative ponds and maturation ponds were −22%, 77% and 35% respectively. Similarly, the results were slightly comparable to the findings reported by Shalaby et al., (2003) who instead reported efficiencies of 3.3%, 63% and 19.5% respectively. TDS concentration increased from influent concentration of 586 mg∙L−l to effluent A1 concentration of 632.5 mg∙L−l. There were fluctuations observed between facultative and maturation ponds. Overall, TDS concentration was increasing and reducing between the different units. Removal efficiencies between the different units were −7.9%, 20% and −6.0% respectively, in anaerobic, facultative and maturation ponds. This study indicates that TDS was not removed by system.
The overall removal of E. coli and total coliforms are shown in
It was observed that overall, total coliforms were not removed in anaerobic ponds as both influent and effluent concentrations were 7.4 log counts (
There was no removal observed for E. coli in anaerobic pond (A1) and the first facultative pond (F1), but instead an increase in concentration of 19.1% was observed. A slight removal was observed at the end of F3 where 8.9% reduction was observed. A pronounced efficiency was observed in maturation ponds where 2.6 log count was observed in the effluent from an effluent concentration of 5.1 log counts. This was a reduction of 49% still revealing that microbes are mostly eliminated in maturation ponds.
Hydraulic efficiency and performance of Palapye waste stabilisation pond system has been investigated. The results from the current study reveal low hydraulic
efficiency as indicated by reduced effective HRT compared to design HRT for each unit in the system. The overall design HRT was 20 days but reduced to effective HRT of 7.1 days. High sludge accumulation in the ponds reduced pond depths therefore HRT resulting in channelling and short circuiting. The overall impact was reduced performance efficiency of the system that does not comply with some of the discharge limits. Despite, all these it was found that total coliforms and some of the physio chemical parameters were within the discharge limit and faecal coliforms were slightly above the limit of 3 log counts. It was also observed that coliforms are mostly removed in maturation ponds. The Palapye treatment system performance efficiency is better but desludging will even improve both hydraulic and performance efficiency.
We also thank Water Utilities Corporation and their Technical Staff for allowing us to conduct research on their facility and assistance rendered.
This work was supported by Botswana International University of Science and Technology through grant initiation programme (grant number 10/2016).
The authors declare no conflicts of interest regarding the publication of this paper.
Gopolang, O.P. and Letshweny M.W. (2018) Performance Evaluation of Waste Stabilisation Ponds. Journal of Water Resource and Protection, 10, 1129-1147. https://doi.org/10.4236/jwarp.2018.1011067