Sewer blockages are on the increase whilst water closet (WC) flush volumes are on the decrease. Furthermore, Water UK reported figures show that the actual number of properties affected by sewer flooding is on the rise. Sewer blockages can lead to sewer flooding of homes and collapse of sewers which impact negatively on social, economic and environmental factors, and therefore, they are not sustainable. Water conservation is required due to water stress but reduced water use results in less water to waste, which in turn reduces solids’ transfer in sewers. When considering reducing water usage through water conservation, these savings could be cancelled out by an increased population and the situation exacerbated by the impacts of climate change. There are issues in relation to varying design methods, a reliance on engineering judgement in sewer design, uncertainty relating to future water stress, and a lack of cross disciplinary design decision-making. Public health engineering solutions are needed to reduce the number of sewer blockages and the environmental impact of sewer flooding. This paper examines the fundamental research that have been carried out in the area of “solid transfer in sewers” resulting from “less water to waste” since the mid-20th Century. Contrary to existing literature, this paper identifies that, now more than ever, this type of research is needed to deal with the increased need for water conservation. To judge that solid transfer research is complete can be compared to supporting a statement that “water conservation is complete”.
The Environment Agency’s (EA) report entitled Less Water to Waste made recommendations [
Swaffield proposed that adequate research on the topic of solid transport had already been conducted [
Wakelin conducted a doctoral study under the supervision of J. A. Swaffield at Brunel University [
The study by Wakelin was undertaken primarily due to the research requirements of the Department of Health and Social Security (DHSS) and aimed to address problems in hospital drainage systems [
The following sub-sections (2.2-2.5) outline the findings of the Wakelin study in terms of solid transfer in low flows [
Wakelin based his research on two brands of WCs―Armitage Shanks and Twyford and used two pan outlet types of each brand (P-trap and S-trap) [
According to Wakelin, flush pipe restrictors are used on the flush pipe from high level cisterns in order to limit splashing from the pan [
Wakelin provided the following reason for not carrying out further tests comparing high level and low level cisterns [
Wakelin also compared the Armitage Shanks and Twyford WCs in terms of the standard pan outlet types available which were P-trap and S-trap, with P-trap providing a horizontal outlet from the pan and the S-trap providing a vertical outlet from the pan both occurring after the initial trap [
Whilst Wakelin looked at both bends and junctions [
Wakelin identified a number of problems associated with the junctions, including water and solids backing-up the main line and solid deposition at the junction [
Wakelin concluded that the “effect of pipe fittings can be ignored in the designed system” if the bend is positioned a minimum of 5.0 m beyond the commencement of what Wakelin refers to as Zone 3 flow conditions [
Subsequently, Littlewood recommended that further research was required to investigate bends and joints [
in pipelines, referred to the need for predefined boundary conditions when using models to predict solid deposition [
Wakelin concluded that venting of the pipeline did not show any significant changes in solid transfer [
Wakelin stated that variation existed between the maternity pads being used in the tests and highlighted that future research programmes must investigate the provision of simulated human faeces [
As with Wakelin’s study, Bokor’s study [
When considering the loading to be used in the tests, Bokor [
Properties of faeces
Bokor examined human faeces simulation and identified the failure to properly simulate human faeces as having delayed drainage research [
Simulated solids
Bokor recommended that any simulated solid would require durability due to friction in order to save “both time and material” and the problems encountered when shaping the simulated solids [
1) Fakazell: this is a product developed by Knoblauch to test WC pans/bowls in terms of their ability to clear the bowl of solids [
2) Flour putty was also tested by Bokor in conjunction with a variety of filler material including papier-mâché, sawdust and polystyrene. Bokor ruled out these mix designs as the simulated solid surface caused the solid to adhere to the wall of the pipe and did not appear to match the observed live waste loadings.
3) Polyvinyl alcohol sponge was trialled. The retention of air in the solid resulted in unpredictability in the testing including the measuring of the specific gravity of each simulated solid.
4) Polyvinyl alcohol powder was also used in mix designs and was discounted for use in Bokor’s testing regime because of the tendency for these solids to be “extremely elastic” which led to a considerable amount of flush failures in terms of evacuation of the WC pan.
5) Potter’s clay was also tested but was eliminated as a possible simulated solid contender because the specific gravity of each mix was greater than 2.0.
6) Bokor also reported on a number of sanitary protection products that were tested.
7) Bokor [
Bokor [
Bokor pointed out that, from personal experience observing trades persons installing pipework on site, sufficient attention to detail was not taken in terms of jointing pipework [
“the movement of the solid was found to be very sensitive to joints and imperfections in the pipe. A displacement of less than 0.5 mm was found to affect the solid transport, particularly when the solid was moving with velocity of less than 0.25 m/s”.
Bokor went on to discuss the significance of the pipe being positioned at the correct gradient throughout [
Bokor made a number of recommendations for further work which focused on WC design, including the discharge geometry from WCs, along with the impact of pipe fittings and varying pipe diameters [
Lillywhite and Webster made significant contributions to the area of determining blockage occurrences in sewers [
Lillywhite et al. studied low water use washdown WCs [
Sewer blockages can happen for a number of reasons. In 2007, it was reported by BBC News that a bra and a pair of knickers were allegedly responsible for blocking a 225 mm diameter sewer in Middleton Saint George, close to Darlington in Co. Durham, England [
Lillywhite and Webster provided results of a site investigation which included two site based studies [
The main finding from the first study was that defective pipe joints were the most common cause of blockage, accounting for 21 (or 30%) out of the total 70 identified faults. One of the main conclusions from the study was that good installation of drains is the most important factor in reducing sewer blockages. Deposits in the line were also identified as being a considerable factor in blockages, accounting for 16 (or 22.86%) out of the total 70 identified faults.
The second study focused mainly on sewer gradients and concluded that the incidence of blockages did not seem to be “seriously” influenced by the sewer gradient. However, this point has been disputed by other researchers including Fenner et al. [
Park and Lee refer to joint type selection as the main cause of joint related problems [
Purcell stated that pipe sizes smaller than 150 mm in diameter are “usually not practical due to the risk of blockage” and also notes this point in foul sewer design examples when considering the adequacy of pipe diameters [
Factor | Apparent increased likelihood % | Is the result statistically significant? |
---|---|---|
Combined pipes are more likely than foul pipes to block | 146 | Yes |
Pipes that surcharge during a 5-year return period design storm | 44 | No |
Pipes that flood during a 5-year return period design storm | 839 | Yes |
Pipes that convey flows influenced by a backwater effect | 55 | No |
Proximity to flow confluence: pipes just upstream of a junction | 38 | No |
Pipes that fail to achieve peak flow velocity of 1 m/s during a 2-year return period event | 81 | Yes |
Pipes laid at a gradient less than 1 pipe diameter | 102 | Yes |
Pipes with large direct inputs | 167 | Yes |
Pipe size―the majority of blocked pipes were ≤225 mm | 521 | n/a |
Blockage type | Company A | Company Ba | ||
---|---|---|---|---|
Total blockages | Percentage | Total blockages | Percentage | |
Damaged pipe | 227 | 3.1 | 38 | 0.6 |
Foreign object | no data | no data | 265 | 3.9 |
FOG | 1552 | 20.8 | 517 | 7.5 |
Sediment | no data | no data | 248 | 3.6 |
Tree rootsb | 5001 | 67.2 | 4981 | 72.4 |
Unknown/other | 664 | 8.9 | 830 | 12.1 |
a represents Company B blockage data from June 2004 to 2008. Between 1999 and June 2004, no cause was logged for the 7922 blockages experienced. b includes FOG-related tree-root blockages.
than 1.0 m). Littlewood reports on causes of blockages and provides information that includes design issues, solids’ deposition and workmanship problems [
Marlow et al. [
On the subject of blockage rates, Arthur et al. (2008) [
Material diameter (nominal) | Company A | Company B | ||
---|---|---|---|---|
Blockages (total) | Blockage rate (per 100 km/year) | Blockages (total) | Blockage rate (per 100 km/year) | |
100 | 235 | 29.59 | 50 | 65.20 |
150 | 6,195 | 15.18 | 13,705 | 23.55 |
225 | 1,015 | 7.12 | 772 | 11.83 |
300 | 42 | 1.69 | - | - |
Marlow et al. reported briefly on a significant trend between periods of drought, when water restrictions were implemented, and an increase in sewer blockages occurring approximately three months thereafter [
• Poor trap design.
• Very flat gradients.
• Excessively large pipes.
• Liberal use of bends, where strictly not necessary.
• Poor inlet design in manholes.
• Manholes containing building material.
• Displaced interceptor rodding eye stoppers.
• Workers laying pipes poorly, particularly with flat gradients.
• Bad jointing of pipes.
• Defective construction of manholes.
• Use of straight pipes for curves.
• The entry of tree roots into pipes.
Littlewood highlighted that one of the possible solutions given by the ISE’s 1954 report was to use smaller pipe diameters and that 75 mm diameter pipes could be considered as an alternative to 100 mm diameter pipes; this was to increase self-cleansing velocities with the condition that “first-class workmanship” be employed for the pipelaying operations [
The study by Fenner et al. was based on 2000 km of sewer network (from 12 separate catchments) and concluded that “shallow slack or moderate” pipe gradients coupled with long pipeline lengths and small pipe diameters increase the probability of sewer blockages [
Rodríguez et al. investigated a sample data set which included more than 248,000 customer complaints about sewer blockage in Bogota, Colombia (covering 7678 km of pipeline) and suggested that the majority of blockages occurred in pipes of between 0.2 m and 0.4 m diameter which were laid at “mild slopes given as between 1/1000 and 1/100” [
Lack of hydraulic capacity has also been identified as a contributory factor in sewers being damaged through surcharge resulting in blockages. Researchers such as Fenner [
Lillywhite et al. investigated WC performance at reduced flush volumes [
Factors identified that can lead to pipe blockages. | Factor identified or cited by author | ||||||
---|---|---|---|---|---|---|---|
Arthur (2008) | Davies (2001) | Fenner (2000) | Lillywhite (1979) | Littlewood (2000) | Marlow (2011) | Rodriguez (2012) | |
Age of sewer | X | X | |||||
Depth of cover | X | X | X | X | |||
Fats, oils and grease | X | X | X | X | |||
Flat gradients | X | X | X | ||||
Joint/material type | X | X | X | ||||
Junctions | X | X | X | ||||
Ground conditions | X | X | X | ||||
Gross solids/misuse | X | X | X | X | X | ||
Intermittent flow | X | X | X | ||||
Length of sewer | X | X | X | ||||
Self-cleansing velocity | X | X | |||||
Solid deposition | X | X | X | ||||
Small diameter | X | X | X | X | |||
Tree roots | X | X | X | X | X | X | |
Quality (construction) | X | X | X | X |
Notes: 1) An X below the author corresponds to a problem identified that can lead to pipe blockages. This means that the author has explicitly written about the factor. 2) The information in the factor identified that can lead to pipe blockages is tabulated based on two or more researchers identifying or citing the same problem. 3) Inadequacies or failures in design have featured among designers as a problem that can lead to an increased probability of blockages. Quality (construction) is used interchangeably with workmanship.
total 25.0 m long pipe replaced with a test rig that allowed observations to be made and water depths to be taken. The Site O test rig, which served two WCs, allowed gradient changes to be made. Both WCs had their flush volumes reduced to 5.25 litres and the WC use was reported to be “very low” with no weekend use. The flush volumes were reduced for experimental purposes to 3.0 litres. Site O was the only site where a blockage was established. The blockage occurred at a gradient of 1/120 as a result of multiple stoppages whilst using a flush volume of 3.0 litres.
The study by Lillywhite et al. included the use of CCTV to check the existing drains before and after the period of reduced flushing volumes [
• There are existing WCs that could be used for flushes of 6 litres without loss in toilet function to clear the bowl or to transfer solids in the drains.
• Solid transfer in sewers could be adversely affected by flush volumes of less than 4 litres. For flush volumes of less than 3 litres, the likelihood of a drain blockage could be increased where there are longer pipe lengths or shallow gradients.
• Some existing WCs could be used for flushes of 5 litres, without loss in standards of cleaning, but most would require some modifications for this size flush.
• It is possible to produce a WC which will give desired function at 5 litres through revised design of the cistern/WC.
• Restrictions on sewer length and gradient may need to be imposed when catering for isolated WCs.
• Cisterns with maximum 7.5 litre volumes are technically available and do not pose a problem to the sewer system or the WC user.
The findings of the study by Lillywhite et al. [
Lillywhite and Webster recommended that further research be carried out in the area of sewer blockages in order to generate informed design guidance for sizing sewers [
There is a myriad of researchers involved in collaborative research in the area of solids’ transfer. One example is Butler who has been involved in several collaborative studies; these studies have included working with Ackers, Ashley, Brown, Davies and Jefferies―some of which will be discussed in this section. Examples of studies that are not specifically discussed in this section are Butler (as doctoral supervisor and part of the work by Babaeyan-Koopaei et al. [
Butler’s PhD was entitled “Modelling the variation of domestic dry weather flow in sewer networks” [
Butler completed a “small scale study of wastewater discharges from domestic appliances” [
The difference in the results between both studies may be influenced by at least two factors, including the time difference between both studies, i.e. Butler (1991) [
Appliance | Sun | Mon | Tue | Wed | Thu | Fri | Sat | Total No. |
---|---|---|---|---|---|---|---|---|
% of the week | ||||||||
WC | 14 | 16 | 14 | 14 | 14 | 14 | 14 | 1956 |
Basin (E) | 11 | 17 | 16 | 15 | 10 | 17 | 14 | 345 |
Basin (R) | 14 | 17 | 14 | 14 | 15 | 13 | 13 | 1463 |
Sink (E) | 15 | 22 | 11 | 12 | 14 | 11 | 15 | 431 |
Basin (R) | 12 | 27 | 14 | 11 | 13 | 12 | 11 | 625 |
Bath (E) | 19 | 15 | 16 | 8 | 12 | 15 | 15 | 124 |
Shower | 13 | 16 | 14 | 14 | 18 | 15 | 10 | 165 |
Washing machine | 16 | 22 | 6 | 7 | 16 | 12 | 21 | 86 |
Notes: R = running to waste; E = Emptying.
It should be noted that Butler compared the results obtained with four other surveys and concluded that this small scale study was reasonably reliable and that useful information can be obtained from small scale surveys.
Butler reported on “the influence of dwelling occupancy and day of the week on domestic appliance wastewater discharges” and also involved 28 households with average occupancy of 2.71 [
Friedler et al. carried out a 7-day diary study of WC usage in 137 households in Southern England [
Friedler et al. reported that “surprisingly, no sanitary towels were reported to be flushed” and went on to explain that, at the time of reporting, the amount of sanitary towels used in the UK was twice that of tampons [
Littlewood [
Refuse | Occurrences | House | Refuse | Occurrences | House | ||||
---|---|---|---|---|---|---|---|---|---|
Count | [%] | Count | [%] | Count | [%] | Count | [%] | ||
Tampon | 78 | 22.9 | 15 | 10.9 | Blotted lipstick | 5 | 1.5 | 2 | 1.5 |
Wet wipes | 49 | 14.4 | 8 | 5.8 | Toe/finger nails | 4 | 1.2 | 3 | 2.2 |
Tissue paper | 40 | 11.8 | 13 | 9.5 | Facial tissue | 4 | 1.2 | 2 | 1.5 |
Cat faeces | 24 | 7.1 | 2 | 1.5 | Makeup | 3 | 0.9 | 3 | 2.2 |
Dental floss | 15 | 4.4 | 5 | 3.6 | Chewing gum | 3 | 0.9 | 1 | 0.7 |
Other non specified | 13 | 3.8 | 7 | 5.1 | Cigarette | 2 | 0.6 | 2 | 1.5 |
Panty liner | 13 | 3.8 | 4 | 2.9 | Spider | 2 | 0.6 | 2 | 1.5 |
Paper towel | 13 | 3.8 | 3 | 2.2 | Sheep tick | 2 | 0.6 | 1 | 0.7 |
Hair | 12 | 3.5 | 7 | 5.1 | Bandage | 1 | 0.3 | 1 | 0.7 |
Tampon applicator | 12 | 3.5 | 4 | 2.9 | Condom | 1 | 0.3 | 1 | 0.7 |
Food | 10 | 2.9 | 9 | 6.6 | J-cloth | 1 | 0.3 | 1 | 0.7 |
Cotton wool | 10 | 2.9 | 4 | 2.9 | Medicated cream | 1 | 0.3 | 1 | 0.7 |
Cotton bud | 8 | 2.4 | 3 | 2.2 | Pot soil | 1 | 0.3 | 1 | 0.7 |
Leaves | 7 | 2.1 | 2 | 1.5 | Soap bar | 1 | 0.3 | 1 | 0.7 |
Vomit (human/animal) | 5 | 1.5 | 3 | 2.2 |
and that this defecation occurs on average once per person per day which closely matches the findings of Friedler et al. [
Friedler et al. [
Along with May, Butler was a co-author in the Ackers et al. study [
The gross solid transport aspect included the use of seven types of NBS solid, ranging in density from 0.96 to 1.06 and ranging in diameter from 10 mm to 38 mm. The study concluded that, based on the seven artificial solid sizes used, solid velocity did not “vary significantly” but stated that “with decreasing size, solids of the same density have been deposited at lower values of depth and velocity” Davies et al. [
A number of recommendations for further work have been made from the studies that Butler has been involved in including:
• Investigation of the effect of steep gradients.
• Development of models to help understand sewer blockage formations.
• Revised sewer sizing through further investigation of domestic peak discharges based on frequency of domestic appliance and WC use per capita.
The lack of available information relating to faeces degradation has also been suggested as a gap in two of the studies in which Butler has been involved. Further recommendations that Butler is associated with are included in Section 7 through collaborations with Littlewood.
Whilst not specifically looking at degradation of human faeces, the following researchers have carried out investigations that mainly related to bed erosion in combined sewers: Ahyerre et al. [
Stotz looked at the factors that affected the first flush in combined sewers and the removal of solids [
Swaffield has collaborated with many other researchers in the area of solids’ transfer in drainage. For example, Swaffield and Marriot looked at the effect of reduced volume WC flushes on how solids were transported in above ground drainage [
• Swaffield and Galowin reported on multi-storey building drainage network design [
• Swaffield and Galowin presented a paper entitled Investigation of apparent limits of drainline waste transport with low volume flush water closets [
• Swaffield and McDougall presented a paper at the International Conference on Sewer Solids entitled Modelling solid transport in building drainage systems [
• Wise and Swaffield co-authored on Water, Sanitary and Waste Services for Buildings [
• Swaffield discussed Transient airflow in building drainage systems [
Given the extent of the overall research collaborations with Swaffield, there is wide scope for review; however, the collaborations reported herein concentrate on the link with Swaffield and LSTD investigations.
Whilst carrying out tests to investigate water conservation, Galowin used plastic cylinders to mimic human faeces [
A concept known as LSTD is referred to by Littlewood [
“Solid velocity depends on surrounding flow conditions. As the flush attenuates solid deposition occurs. This implies that for one w.c. in one pipe of fixed slope, diameter and roughness and where there is no other supporting flow, there is a MAXIMUM travel distance for any solid regardless of how many times the w.c. is flushed”.
This statement was made showing a 150 mm diameter sewer at a gradient of 1/100.
McDougall and Swaffield presented research on the influence of water conservation on drain sizing for building drainage systems [
solid as shown in
Cummings, Swaffield and Wakelin presented research on hydraulic assessment of non-circular-section building drainage [
Whilst reference is made to ovoid or “egg shaped” sewers within the research, none of the non-circular concepts including the parabolic gutter match the geometry of either the standard (wide or metropolitan egg) or the new (narrow
egg). However, the parabolic gutter is a similar concept to the egg-shaped pipe approach. The paper demonstrated that non-circular pipes can give increased solid travel distances of more than 50% at gradients of 1/100. Whilst the paper did not specifically call for further research in the area of non-circular pipes, it is implied by making reference to the need to research innovative drainage networks, with increased travel distances, illustrating that this is a significant area where research is required. An increase in the use of wipes, due to a change in personal hygiene habits and subsequent flushing through sewer systems, is explicitly stated as an area that should be researched in the future.
In terms of the use of non-circular pipes for sanitation, there have been parabolic pipes found in Knossos, Crete that date back to 1600 BC, according to Bertrand-Krajewski [
Jack and Swaffield state that “efficacy and efficiency in drainage provision need not be subjected to ‘over-design’, i.e. through excessive water use” [
In Cummings et al. [
The aim of Littlewood’s study was to research if the reduction in pipe diameters could increase carrying capacity of flush waves when considering intermittent flows. The study also considered if the diameter reduction would increase pipe blockages and was entitled “Movement of Gross Solids in Small Bore Sewers”. The study resulted in a computer model of solid movement under intermittent flow. The model was verified by an extensive programme of practical experiments. Results derived from the model were used in the WRc review of Part H of the Building Regulations. Littlewood [
Among the variety of tests which Littlewood [
Ashley et al. [
• Increased flush volumes provide increased toilet paper velocity and flush distance.
• The velocity and distance travelled of 10 sheets was less than that of 5 sheets of toilet paper.
• There was no LSTD established within the 25 m length of the test rig/pipe.
• Wave intensity does not necessarily affect toilet paper.
• Smaller pipe diameter provides increased toilet paper velocity and flush distance, with more predictable results.
The main findings for combination solids by Littlewood [
• Solid movement is significantly influenced by the size of the toilet paper dam.
• Westminster solid travel distances increase when toilet paper is added to a flush.
• Large Westminster solids (as part of a combination) move with a lower velocity than those with smaller Westminster solids and with reduced distance of solid travel for equal volume flushes.
• Higher specific gravity combination solids reduce the distance of solid travel for equal volume flushes.
Reference code | Length (mm) | Diameter (mm) | Specific gravities (r/rw) | Colour |
---|---|---|---|---|
LW | 80 | 38 | 1.05 | white |
MW | 44 | 20 | 1.05 | white |
SW | 22 | 10 | 1.05 | white |
LB | 80 | 38 | 0.97 | black |
MB | 44 | 20 | 0.97 | black |
SB | 22 | 10 | 0.97 | black |
• Larger flush volumes increase distances of solid travel.
• Smaller pipe diameters increase the initial solid velocity and the distance moved for a particular solid/wave combination.
• The solid movement mechanism for a combination of solids depends on the amount of water (if any) behind the solid.
Littlewood suggests that a deviation in flow depth of “as little as 1 mm” could cause a solid to deposit [
Littlewood [
Littlewood [
Ω = A s ∗ A f
The shape factor is illustrated in
where
A s ∗ = solid end area occupying the flow area.
Af = total flow area.
Littlewood found that shape factors of 50% - 80% were common in larger pipe diameters, and that high shape factors of 95% were less likely to occur in larger diameter pipes [
upstream solid area thus increasing the force on the solid due to increased water being dammed behind the solid, therefore creating conditions to assist with the increasing of solid transfer distances. Littlewood also highlighted the “sliding leaking dam” that relates to solid movement with varying quantities of the dammed water (behind the solid) leaking past the solid while the solid is sliding along the invert of the pipe [
Littlewood found that the LSTD increased for the same flush and given solid combination through a decrease in pipe diameter [
Littlewood [
• The effect of junctions and bends to assess solids’ movement with a sliding, leaking dam mechanism.
• Further testing on clay pipes to determine the effect of pipe materials on solid movement.
• Investigating the coalescing of multiple solids in the sewer including the effect on LSTD.
• Self-cleansing velocity in relation to maximum distances of pipes design relating to the sliding, leaking dam mechanism zones.
• The effect of pipe gradient on the movement of solids within the sliding, leaking dam zone.
Whilst the majority of low flush rig-based testing has been carried out in the UK, a Canadian researcher (Bill Gauley) has carried out rig-based testing to determine the impact of reduced flushes. Gauley has also trialled a number of different materials in order to simulate human faeces. Gauley has carried out the testing of WCs in terms of efficiency and the ability to evacuate solids from the bowl. This work is still ongoing and is wide ranging because of changes in WC flush technologies and cistern design.
A significant amount of work has been completed by Gauley in relation to testing the performance of WCs. Gauley and Koeller refer to this work as Maximum Performance (MaP) testing [
Gauley and Koeller produced a report called the Evaluation of Low-Flush-Volume Toilet Technologies to Carry Waste in Drainlines [
Gauley makes reference to the work of Swaffield within the 2005 report and draws comparisons between the Swaffield and Galowin study by showing that the flush travel distance is more affected by pipe gradient than pipe diameter [
Unassisted flushing technology (gravity only) provides the least impact in terms of evacuation of the bowl when compared with power assisted. The reason for choosing gravity only in the research is that gravity wash-down WCs are the most popular in the UK. By comparing Gauley’s earlier MaP testing of toilets and the low flush testing, it was confirmed by Gauley that there was no correlation between the ability of a toilet to evacuate a large mass of solids and flush distance achieved as shown in
The Gauley and Koeller [
Gauley and Koeller [
• Other flows such as washing machines, dishwasher, sinks and dishwashers.
• The effects of different types of pipes including cast iron salvaged piping from buildings that were going to be demolished.
• Testing of sanitary products.
• Carrying out the tests using toilets that are only capable of evacuating 200 g or less from the bowl.
• Testing using different test media.
• Allowing the media to dry in the line to simulate weekend or vacation periods.
A number of rig-based research projects have been completed in the past decade. The EA has also reported on a number of rig-based research, the most significant of these are summarised herein.
Flushing technology | Average carry distance (m) | MaP testing results (g of evacuated material) |
---|---|---|
Pressure-Assist―6.0-L | 20.1 | 900 |
Gravity―Rim Jet | 16.3 | 550 |
Gravity | 14.9 | 900 |
Gravity―Siphon Jet in Sump of Bowl | 14.8 | 325 |
Gravity―Siphon Jet in Trapway | 14.0 | 375 |
Gravity―Tipping Bucket | 15.0 | 725 |
Gravity―Washdown | 10.9 | 650 |
Gravity―Vacuum-Assist | 8.1 | 500 |
The EA report entitled Less Water to Waste considered existing rig-based testing research that had been carried out [
The EA report agrees with Littlewood [
Gormley and Campbell reported on the transport of solids in above ground drainage using a rig which tested 14.0 m of pipe (between loading junctions) to develop a model to predict a surge wave in a pipe [
The Plumbing Efficiency Research Coalition (PERC) produced The Drainline Transport of Solid Waste in Buildings report which used a test-rig consisting of a total length of 41.0 m of 100 mm diameter clear plastic pipe [
PERC [
“We highlight Professor Swaffield’s presentation, inasmuch as the thesis of his presentation was that adequate research on the topic of drainline transport had already been conducted (implying, therefore, that further such research was unwarranted). Later, this thesis was repeated in another paper and presentation he provided for the 2009 CIB-W062 conference in Düsseldorf, Germany, in which he specifically referred to the recently announced PERC Drainline study effort as unnecessary”.
PERC [
Aspects of the PERC [
Variable or factor reference No. | Variables that influence solid transport and factors identified that can lead to pipe blockages | No. of authors agreeing with the variable or factor | Author and year |
---|---|---|---|
Significant variables (PERC 2012) | |||
SV1 | Pipe gradient | 3 | Flat gradients are identified by Arthur et al. [ |
SV2 | Toilet paper used | 4 | Toilet paper has been identified by Littlewood [ |
SV3 | Flush volume | 3 | Flush volume is a factor in solid transport, by implication a factor in solid deposition which is identified by Arthur et al. [ |
Non-significant variables (PERC 2012) | |||
NSV1 | % of trailing water | 0 | In the blockage factors % trailing water has not been explicitly provided as a cause of blockages; however, |
NSV2 | Flush rate (velocity) | 2 | Flush velocity is related to self-cleansing velocity, self-cleansing velocity has been identified by Arthur et al. [ |
Blockage factors from | |||
BF1 | Age of sewer | 2 | Davies et al. [ |
BF2 | Depth of cover | 4 | Davies et al. [ |
BF3 | Fats, oils and grease | 4 | Fats, oils and grease have been identified by Davies et al. [ |
BF4 | Joint/material type | 3 | Davies et al. [ |
BF5 | Junctions | 3 | Junctions have been identified by Arthur et al. [ |
BF6 | Ground conditions | 3 | Davies et al. [ |
BF7 | Gross solids/misuse | 5 | Arthur et al. [ |
BF8 | Intermittent flow | 3 | Arthur et al. [ |
BF9 | Length of sewer | 3 | Arthur et al. [ |
BF10 | Small dia. pipe | 4 | Arthur et al. [ |
Notes: SV = Significant Variable 1; NSV = Non Significant Variable; BF = Blockage Factors.
listed against the significant and non-significant variables from PERC [
There appears to be a wide range of variables that can affect solid transport, thus possibly causing sewer blockages even when excluding obvious causes such as tree roots (identified by six researchers) and quality of construction (identified by four researchers) which have been excluded from
The PERC report [
Gap ID | Area researched | Researchers and areas that they have investigated | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Bokor | Butler | Cummings | Gormley | Gauley | Lillywhite | Littlewood | PERC | Swaffield | Wakelin | ||
1.0 | Forces on solids | ü | ü | ü | ü | ü | |||||
2.0 | Influence of TP | ü | ü | ü | |||||||
3.0 | LSTD | ü | ü | ü | |||||||
4.0 | WC usage behaviour | ü | ü | ü | |||||||
5.0 | WC type | ü | |||||||||
6.0 | WC flush volume | ü | ü | ||||||||
7.0 | Pipe configuration | ü | |||||||||
8.0 | Pipe dimensions | ü | ü | ü | ü | ü | ü | ü | ü | ü | ü |
9.0 | Pipes (non-circular) | ü | |||||||||
10.0 | Pipe gradient | ü | ü | ü | ü | ü | ü | ü | ü | ü | ü |
11.0 | Simulated faeces | ü | ü | ü | ü | ||||||
12.0 | Faeces degradation | ü |
Note: TP = Toilet Paper.
Water conservation is critical in order to assist in providing sustainable development in a UK context. When considering climate change, in terms of the availability of raw water, there is uncertainty for the UK population which is predicted to increase from 63.7 million in 2012 to 73.7 million in 2037 [
Along with water conservation being critical, continued research addressing the effects that less water to waste has on solid transfer in sewers is critical.
1) Production of a “Simulated Solid” to mimic human waste for testing Sewers: this study shows how human faeces are tested to allow development of a simulated solid that mimics human faeces.
2) Performance of non-circular sewers in delivering reduced water consumption: this study reports on the performance of small diameter “egg shaped” sewers.
3) Degradation of human faeces in sewers: this study reports on sewer degradation mechanisms associated with human faeces transfer in foul sewers.
Gap ID | Gap identified | Researchers and the research gaps identified | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Bokor | Butler | Environment agency | Gormley | Gauley | Lillywhite | Littlewood | PERC | Swaffield | Wakelin | ||
1.0 | Effect of junctions and bends | x | x | x | x | ||||||
2.0 | Quality/installation errors | x | x | x | x | ||||||
3.0 | Simulated faeces | x | x | x | |||||||
4.0 | Steep/variable gradients | x | x | x | |||||||
5.0 | Impact of wipes on sewers | x | x | ||||||||
6.0 | White goods | x | x | ||||||||
7.0 | Single occupancy dwellings | x | x | ||||||||
8.0 | Baths versus showers | x | |||||||||
9.0 | The use of non-circular pipes | x | |||||||||
10.0 | Degradation of human faeces | x | |||||||||
11.0 | Surfactant dosing | x |
Notes: Gap No. 5 should also look at “flushable” products and should define what “flushable” means. Gap No. 10 degradation of human faeces identified through collaborations Butler had within Davies et al. [
4) Revised foul sewer design for low flows: this study proposes revised foul sewer design guidance to cater for less water to waste.
5) Impact of single occupancy & vacation periods on sewer blockages: this study reports on a 6-month simulation of houses with varying occupancy levels.
Whilst the above 5 research studies each address a gap in the current literature in the area of “low flow sewers”, there is still a significant scope for future research to address all gaps identified by this paper.
A cross-party report has urged the UK Government to champion water efficiency in the built environment and specified a 100-litre water usage per person per day to make green infrastructure “the norm” [
Solid Transfer in Low Flow Sewers, the Distance Travelled in terms of research so far is not enough. To judge that solid transfer research is complete can be compared to supporting a statement that “water conservation is complete”.
The authors declare no conflicts of interest regarding the publication of this paper.
McDermott, R., Strong, A. and Griffiths, P. (2019) Solid Transfer in Low Flow Sewers, the Distance Travelled So Far Is Not Enough. Journal of Environmental Protection, 10, 164-207. https://doi.org/10.4236/jep.2019.102011