Variability of Settling Characteristics of Solids in Dry and Wet Weather Flows in Combined Sewers: Implications for CSO Treatment
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- Krishnappan, B.G., Exall, K., Marsalek, J. et al. Water Air Soil Pollut (2012) 223: 3021. doi:10.1007/s11270-012-1085-9
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Four devices developed for measuring settling velocity distributions of combined sewer overflow (CSO) solids were applied to dry and wet weather flow samples from an urban area serviced by combined sewers (Welland, ON, Canada). The settling column-based methods (the Aston, Brombach and US Environmental Protection Agency columns) produced comparable results indicating minimal differences in settleability of dry and wet weather samples. The elutriation apparatus, which assessed settling velocities in a flowing medium, indicated higher settleabilities than the column methods. This was attributed to enhanced opportunities for particle coalescence in the flowing medium, which should better approximate actual sedimentation conditions. While the elutriation apparatus also indicated larger differences in settleabilities between dry and wet weather samples than the column methods, this difference was not statistically significant. Experimental distributions of particle settling velocities were approximated by a mathematical function, which was then used to estimate partial settling of total suspended solids (TSS) with settling velocities smaller than the clarifier overflow rate. The TSS removal target of 50%, which is applicable to CSOs in Ontario, could be met for overflow rates ranging from 4.7 to 6.8 m/h, for dry and wet weather flows, respectively, based on the average settling velocities measured. Experimental data collected in the study indicate that the design of CSO storage and settling facilities is affected, among other factors, by both the apparatus used to assess CSO settleability and the inter-event variability of CSO settling characteristics.
KeywordsCombined sewer overflow (CSO)Suspended solidsSettling columnElutriation apparatusSettling velocity distribution
Combined sewer overflows (CSOs) are a major source of intermittent pollution impacting the receiving waters in the Great Lakes region as well as in many other urban areas serviced by combined sewers. To mitigate such pollution, a number of approaches have been developed, including the following: full or partial sewer separation; reduction of stormwater inflow into combined sewers by stormwater management; implementation of storage and conveyance facilities serving to retain CSOs until treatment capacity becomes available to process the stored flow at a satellite CSO plant or at the central sewage treatment plant (STP); and real-time control systems serving to optimise operation of the overall system. Some of these options require concurrent expansion of the central STP to treat the intercepted overflows. The choice of solutions depends mostly on the regulatory environment (design criteria), water quality objectives for receiving waters, and cost considerations, and broadly varies from one jurisdiction to another, as noted by Zabel et al. (2001).
It was reported by Zukovs and Marsalek (2004) that in Canadian municipal pollution control practice, increasing attention has been paid to satellite CSO treatment, which often represents a cost-effective solution to the CSO problems. In Ontario, CSO treatment and control is governed by Procedure F-5-5 (MOE undated), which specifies seven minimum CSO controls, including capture and treatment of 90% of the wet weather flow volume during a 7-month period (approximately April – October) in an average precipitation year, with a level of treatment equivalent to primary treatment. Effluent guidelines include removals of 50% and 30% of total suspended solids (TSS) and carbonaceous biochemical oxygen demand (C-BOD5), respectively, with treated TSS concentrations not to exceed 90 mg/L more than 50% of the time at satellite treatment facilities. The combination of volumetric control and treatment is best achieved in storage tanks with settling (gravity separation), and where such a technology does not provide an adequate level of treatment, other options for removal enhancement are available.
The design of a CSO storage and treatment facility is governed by a number of factors, among which the most important are (a) criteria for CSO control, (b) choice of design rainfall data, (c) CSO composition and settleability, and (d) hydraulic design of the facility. Focusing on Ontario conditions, the first factor is described in the F-5-5 Procedure (MOE undated) and further guidance can be found in Zabel et al. (2001) or national guidelines; the choice of rainfall data is defined in F-5-5 as a historical 20-year (or longer) rainfall time series, and general aspects of requirements on rainfall data were discussed, e.g., by Rauch et al. (1998); CSO composition is best determined by local sampling, and settleability is addressed by the methods discussed below; and, finally, the hydraulic design is best addressed by CFD modelling as reported by He et al. (2006). Thus, further discussion focuses on TSS removal and CSO settleability.
Gravity separation provides a passive TSS removal option that has been used extensively in primary treatment of wastewater and can be applied in the treatment of CSOs in settling tanks. Among the parameters that govern the removal of solids in settling tanks, CSO treatability, usually described by settling velocity distribution of TSS, is recognised as the most important one.
Settling velocity distributions of solids in CSOs are usually determined experimentally using different apparatuses and procedures, which can be divided into two categories (Marsalek et al. 2006): (a) devices with the settling medium at rest (i.e., various types of settling columns) and (b) devices mimicking dynamic settling conditions by using flowing medium, or subjecting it to mechanically generated turbulence. The first category clearly prevails and includes most of the apparatuses used in the past (e.g., see Pisano and Brombach 1996), including different versions of the conventional settling column such as the Aston column (Tyack et al. 1993), Brombach column (Michelbach and Wöhrle, 1993), US Environmental Protection Agency (EPA) column (O’Connor et al. 2002; Piro et al. 2011), Andreasen pipette and a settling column developed by Institut de Filtration et des Techniques Separatives, France (Aiguier et al. 1996), and the newer VICAS and VICPOL protocols (Gromaire et al. 2008). In the second category, one can name devices with turbulence generators (Dobbins 1944; Rasmussen and Larsen 1996) and the elutriation apparatus described by Krishnappan et al. (2004).
Comparative evaluations of various settling velocity measurement devices have been carried out by several investigators (e.g., Aiguier et al. 1996; O’Connor et al. 2002; Marsalek et al. 2006). Aiguier et al. (1996) compared four designs of settling columns (i.e., the Aston column, Brombach column, Andreasen pipette and another settling column proposed in France) and observed that the various columns produced different settling velocity distributions. O’Connor et al. (2002) arrived at similar conclusions when comparing the US EPA column results with those produced with the Aston column. Marsalek et al. (2006) compared the US EPA column with the elutriation apparatus that was first developed by Walling and Woodward (1993) and later adapted by Krishnappan et al. (2004) for CSOs. Again, they noted differences in settling curves produced under static and dynamic conditions. Thus, the experimental assessment of CSO settleability is subject to uncertainties introduced by the use of various devices and methods, and such uncertainties may be of the same order of magnitude as the inter-event variability of the CSO samples tested using a single method. Such observations led to the conclusion that the choice of a measurement method for characterising the size distribution of CSO samples should not be governed solely by its ability to reproduce the field condition more closely but also by the method’s attributes, such as ability to address settling with chemical aids, testing/capture of floaters and practicality (Marsalek et al. 2006).
The main objectives of this paper are to examine variability in the CSO settleability assessment by four methods, including the Aston (Settling) column (ASC), the Brombach column (BRC), the US EPA column (EPAC) and the elutriation apparatus (ELA), and to compare this to the inter-event variability. Such considerations are of utmost importance when designing CSO storage and treatment facilities expected to meet specific removals of TSS.
2 Methods and Study Area
2.1 Settling Velocity Measurement Devices
A detailed description of the various settling velocity measurement devices and methods can be found in Exall et al. (2005); brief descriptions are included here for the sake of completeness.
2.1.1 Aston Column
2.1.2 Brombach Column
The Brombach column (also known as the UFT, or Umwelt- und Fluid-Technik, column) has been used extensively in Germany to characterise the settleability of CSOs, and such data were used to design CSO storage tanks (Michelbach and Wöhrle 1993). The column consists of an upper reservoir (500 mL), with an offset sample delivery cylinder, a middle, transparent column section (approximately 5 cm ID × 49 cm) and the lower portion of an Imhoff cone (100 mL) attached to the bottom of the column (see Fig. 1b). In the BRC procedure, a 1-L sewage sample was pre-settled in the column for 2 h, and the settled solids were collected. The column was then drained and refilled with tap water, and the settled solids were reintroduced at the top of the column. Sub-samples were withdrawn from the bottom of the column at the following timed intervals: 7 s, 14 s, 28 s, 56 s, 112 s, 225 s, 7.5 min, 15 min, 30 min, 60 min and 120 min.
2.1.3 US Environmental Protection Agency Column
The US EPA column is also known as the “long” column and was described in general terms by O’Connor et al. (2002). It is usually constructed of clear acrylic and fitted with evenly spaced side ports for sample withdrawal and a drain valve at the bottom for emptying the column. The column used in this study was 1.5 m long and 0.127 m in diameter with an approximate volume of 20 L and had ports spaced at 0.253 m (see Fig. 1c). A well-mixed sewage sample was rapidly poured into the column at the start of the EPAC test and sub-samples were withdrawn from the ports at the following timed intervals: 2, 4, 8, 16, 30, 60, and 120 min.
2.1.4 Elutriation Apparatus
The original elutriation apparatus was developed by Walling and Woodward (1993) to measure particle size distribution of riverine suspended sediment and consisted of four cylinders with inner diameters of 25, 50, 100 and 200 mm, which were arranged sequentially in the ascending order of their diameters. The river water was drawn through these cylinders by a pump placed at the downstream side of the cylinders. The water was routed through these cylinders in an upward direction, so the river sediment with a settling velocity higher than the upward velocity of the water would settle in a particular cylinder. Since the diameters of the cylinders were progressively increasing (and the corresponding flow velocities decreasing), sediment with different settling velocities settled in different cylinders. By measuring the amount of sediment in each cylinder, the settling velocity distribution was deduced.
2.2 Study Area and Sample Collection
Experimental investigations focused on both dry and wet weather flow samples collected at the inflow to the Welland Wastewater Treatment Plant located in the City of Welland, ON, Canada. The City has a population of about 60,000 and has been pursuing sewer separation to reduce basement flooding and CSOs into the Welland River. Currently, the City is planning to implement CSO treatment, and hence, there is interest in assessing the settleability of CSOs. The plant has an average daily flow of 37,500 m3/d (0.434 m3/s) (Zukovs et al. 2005). For operational purposes, the wet weather (WW) flows were defined as those exceeding a plant inflow of 55,000 m3/d; smaller flows were considered as dry weather (DW) flows. The ASC, BRC and EPAC were all tested for 12 DW and 13 WW events between July 2003 and July 2004, and the ELA was tested for seven DW and 12 WW events over that time period. Because of insufficient sample volumes, the four settling devices were not tested for all of the same events, which brings some additional uncertainties into comparisons of the devices.
Wet weather samples were collected by automatic samplers programmed to collect about 90 L of CSO during the first 2 h of overflow; the same volume was collected over 20 min using a submersible pump during dry weather sampling (Exall et al. 2005). Samples were transported to the laboratory in Burlington, where they were stored in the dark at 4°C until tested, generally within a period of 1– 4 days. Just prior to testing, the samples were brought to room temperature (20°C), mixed/homogenised by continuous pumping through a 0.37 kW (1/2 hp) pump (Q = 300 L/min.) and divided for analysis by the different test methods. At the start of each test, a whole-water sample of the well-mixed sewage was withdrawn and tested for TSS to determine total solids in the raw sample at the start of the test.
3 Results and Discussion
The presentation and discussion of results starts with the display of settling velocity distributions measured by individual devices, followed by presentation of an empirical relationship approximating the velocity distribution and derivation of a per cent TSS removal relationship, which was used to demonstrate the effect of inter-event variability on TSS removals.
3.1 Settling Velocity Measurements
3.1.1 Aston Column
3.1.2 Brombach Column
The results obtained with the Brombach Column are somewhat similar to those obtained with the Aston Column. Again, the cumulative distributions of settling velocity showed a significant scatter, the magnitude of which varies as a function of the settling velocity: higher inter-event variation corresponds to lower settling velocities and vice versa. This scatter is described by the standard deviation, which ranged from 0.24 to 11.46% for dry weather samples and from 0.10 to 14.70% for wet weather samples. Compared to Aston Column results, the Brombach Column gives similar standard deviation values for the lower end of the settling velocity range but shows a lower scatter at the higher end.
The mean settling velocity distribution curves given in Fig. 5 show that the Brombach Column is no better in distinguishing the differences between the dry and wet weather samples than the Aston column (see Fig. 4).
3.1.3 US EPA Column
The settling velocity distributions obtained with the EPAC show larger scatter than observed with the ASC or BRC methods. The mean distribution given by this method appears to be quite different than the mean distributions produced with the Aston and Brombach methods, but it should be noted that the range of settling velocities measured with the EPAC is shifted to lower values when compared to the other methods. As seen with ASC and BRC, the differences between the results for dry and wet samples given by the US EPA method are negligible.
3.1.4 Elutriation Apparatus
While these results, in general, may appear comparable to those obtained by the other three methods, there are some significant differences. The main difference is in the higher settling velocities measured for both wet and dry weather samples, compared to those measured using the static column methods. The mean curves also show that the settling velocities of solids in dry weather samples are somewhat lower in comparison to the solids in wet weather samples, although the difference is not statistically significant.
Although it is generally accepted that details of the experimental procedure may influence the measured settleability of a sample (Chancelier et al. 1998), it is interesting to note that the BRC and EPAC methods produce very similar settling velocity curves. This is in spite of the fact that the BRC method requires initial separation of settleable particles and measurement from a “floating layer”, as compared to the EPAC method, which measured solids settling from a fully mixed or homogeneous sample.
The higher settleabilities observed using the elutriation apparatus can be attributed to the greater chance of particle coalescence in a flowing medium, which better simulates field conditions. In an earlier study carried out by Krishnappan et al. (2004), it was shown that the elutriation apparatus was also ideally suited for testing the effectiveness of chemical aids (coagulants and flocculants) for the promotion of enhanced settling of CSO solids in sedimentation tanks, particularly when a static mixer was incorporated at the front end of the system.
The main drawback of the elutriation apparatus is that it requires a larger sample volume (about 50 L) per test. The set-up and take-down of the elutriation test are also inherently more involved, due to the number of glass cylinders employed. The sample volume and method complexity could be reduced by decreasing the number of cylinders in the set up, but this would reduce the detail of the resulting settling velocity distributions.
3.2 Empirical Relationship for Evaluating the Total Removal of TSS from CSOs by Settling
Further analysis of experimental data focused on approximating the cumulative settling velocity distribution of solids in CSOs by a mathematical function and using such an approximation to assess the total removal of TSS from CSO samples by settling.
Mean values of α and β in Eq. 1 (with 95% confidence intervals in parentheses) for dry and wet weather samples analysed with the four settling methods
The above equation gives the percentage of solids that would settle in a sedimentation tank for a given overflow rate Vc, in terms of the empirical coefficients that define the cumulative settling velocity distribution of the CSO solids. By measuring the cumulative settling velocity of a CSO sample using a device such as the elutriation apparatus and fitting the measured distribution to the empirical equation as given in Eq. 1, the percentage of the solids that would settle in a sedimentation tank for a given overflow rate Vc can be computed using Eq. 4 or 5.
The procedure introduced herein can be used for designing CSO settling facilities. In the first step, the settleability of CSO samples is tested using a settling velocity measurement device (e.g., one of those described in Section 2.1). Note that the settling velocities measured by these devices are considered in practice as applicable in the full scale facilities, without any scaling. In the second step, settling velocity distributions (examples of which are displayed in Figs. 3-7) are approximated by Eq. 1, and the fitted values of coefficients α and β are determined. Finally, the percent removal is calculated by Eq. 5. An example of such an application follows.
As stipulated in the earlier introduced Procedure F-5-5 (MOE undated), on average, a 50% TSS removal needs to be achieved in CSO settling. For data presented in Fig. 9, this would be achievable in wet weather flow conditions with an overflow rate of up to 6.8 m/h (1.9 mm/s). Such a value is somewhat higher than the values of 1.67 m/h (referred to as a typical value) and 4.2 m/h (peak hourly flow) recommended in Metcalf and Eddy (2003) for primary sedimentation tanks followed by secondary treatment. Recognising that F-5-5 refers to primary treatment equivalent and average removals, the adoption of the average α values should be acceptable, assuming that the second F-5-5 condition, TSS not exceeding the limit of 90 mg/L more than 50% of the time, is met. Where a more robust design would be required, smaller values of α should be considered, perhaps as small as the lower 95% confidence limit (an overflow rate of 3.9 m/h). Alternatively, better performance could be obtained with chemically-aided settling (Krishnappan et al. 2004), however, at higher operating costs.
4 Summary and Conclusions
Settling velocity distributions of dry and wet weather flow samples collected from combined sewers were determined by four devices, which were developed for measuring settleability of CSO solids. The measured distributions indicated variability, which depended on the device and method used and the sampled event characteristics. In general, the static settling column-based methods (the Aston, Brombach and US EPA columns) produced comparable results, indicating barely noticeable differences in settleability of dry and wet weather samples. The fourth method, the elutriation apparatus, assessed settling velocities in a flowing medium and indicated higher settleabilities than the column methods. This difference was attributed to enhanced opportunities for particle coalescence in the flowing medium which better approximated field conditions. The scatter of experimental points for individual events about the mean settling velocity distribution curves (which are often used in design) was generally characterised by low standard deviations (<5%) for higher settling velocities (Vs > 6 mm/s) and substantially higher standard deviations (10–20%) for low settling velocities (<2 mm/s). Experimental distributions of particle settling velocities measured with the elutriation apparatus were approximated by a mathematical function, which was then used to estimate partial settling of TSS with settling velocities smaller than the clarifier overflow rate. The provincially applicable TSS removal target of 50% could be met for overflow rates ranging from 4.7 to 6.8 m/h, for dry and wet weather flows, respectively, and assuming average settling velocity distribution curves. Experimental data collected in the study indicate that the design of CSO storage and settling facilities is affected, among other factors, by both the equipment used to assess sewage settleability and inter-event variability of CSO settling characteristics.
We would like to acknowledge the collaboration of a number of partners who assisted us in this study. XCG Consultants conducted the wet-weather sampling and provided field support, the City of Welland, Ontario provided access to their facilities, flow data and partial funding for this project, the Great Lakes Sustainability Fund for providing funding for this project and Dr. Peter Seto, who facilitated the initiation of this research project and provided technical guidance.