Quantity and Quality Benefits of In-Service Invasive Cleaning of Trunk
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Drink. Water Eng. Sci., 10, 45–52, 2017 https://doi.org/10.5194/dwes-10-45-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Quantity and quality benefits of in-service invasive cleaning of trunk mains Iftekhar Sunny1, Stewart Husband1, Nick Drake2, Kevan Mckenzie2, and Joby Boxall1 1Pennine Water Group, Department of Civil and Structural Engineering, University of Sheffield, S1 3JD, Sheffield, UK 2Scottish Water, Juniper House, Heriot Watt Research Park, Edinburgh, EH14 4AP, UK Correspondence to: Iftekhar Sunny (izsunny1@sheffield.ac.uk) Received: 24 January 2017 – Discussion started: 1 February 2017 Revised: 25 May 2017 – Accepted: 1 June 2017 – Published: 6 July 2017 Abstract. Trunk mains are high risk critical infrastructure where poor performance can impact on large numbers of customers. Both quantity (e.g. hydraulic capacity) and quality (e.g. discolouration) of trunk main performance are affected by asset deterioration in the form of particle accumulation at the pipe wall. Trunk main cleaning techniques are therefore desirable to remove such material. However, little is quantified regarding the efficacy of different maintenance interventions or longer-term changes following such cleaning. This paper presents an assessment of quantity and quality performance of a trunk main system pre, post and for 12 months follow- ing cleaning using pigging with ice slurry. Hydraulic calibration showed a 7 times roughness height reduction after ice slurry pigging, evidencing substantially improved hydraulic capacity and reduced headloss. Turbid- ity response due to carefully imposed shear stress increase remained significant after the cleaning intervention, showing that relatively loose material had not been fully removed from the pipe wall. Overall the results demon- strate that cleaning by pigging with ice slurry can be beneficial for quantity performance, but care and further assessment may be necessary to realise the full quality benefits. 1 Introduction frastructure performance assessment both for pre-post clean- ing water quality and quantity are vital to justify the invest- ment. One of a water company’s primary responsibilities is to op- Quantity performance, e.g. hydraulic capacity and pipe erate and maintain their distribution network performance roughness, can impact on the quantity of water received by to ensure the continuous supply of safe, high water quality. the consumer, fire flow capacity and energy (pumping) costs. As part of the drinking water network, transmission (trunk) Continuous fouling, tubercles and scaling can increase hy- mains are categorised as critical infrastructure as poor perfor- draulic resistance and reduce effective pipe diameter (Boxall mance can impact large numbers of downstream customers. et al., 2004). Microbial activity on the pipe wall can also raise Due to strict operational and quality regulations and con- boundary resistance and affect hydraulic capacity (Cowle et cern regarding potential consequences, UK water companies al., 2014). have tended to avoid operational activities associated with Discolouration is the water quality issue most apparent to trunk mains (Husband and Boxall, 2015). However, this is the customer, causing the highest contact rates (i.e. number becoming unavoidable and water utilities rehabilitation pro- of contacts per 1000 population per year) worldwide. Parti- grammes now include large undertakings for the cleaning of cle size analysis demonstrated that discolouration processes large diameter mains (i.e. trunk mains) to manage asset re- are dominated by small particulate material (2–25 µm); thus, silience and reduce water quality risks. Considering the mas- their behaviour is unlikely to be dominated by gravity driven sive total expenditure allocated for cleaning interventions, in- Published by Copernicus Publications on behalf of the Delft University of Technology. 46 I. Sunny et al.: Quantity and quality benefits of in-service invasive cleaning of trunk mains Figure 1. (a) Trunk main layout, monitoring points with elevation profile, and (b) cleaning intervention planning and timelines. processes, but rather by continuously and ubiquitously at- 2 Methodology taching to the pipe wall by a complex combination of phys- ical, chemical and biological processes (Boxall et al., 2001). 2.1 Selected network characteristics The internationally validated PODDS (Prediction of Dis- colouration in Distribution Systems) model was developed The selected trial trunk main was 2.4 km of 228 mm inter- and later verified that these particles are structured in sta- nal diameter (D) asbestos cement (AC) pipe. In this case, ble cohesive layers and are released into the bulk water once the trunk main was defined by an operation providing supply an excess shear is imposed (Boxall and Saul, 2005). Recent from a service reservoir into downstream distribution zones, studies have indicated that biofilms are a key component of with no consumers directly connected. The supplied DMAs the discolouration process. facilitating inorganic particle ab- (district metered areas) were mostly residential with consis- sorption within the organic matrix (Douterelo et al., 2014; tent demand across the year with sub-daily variations. The Husband et al., 2016). PODDS concepts are well accepted downstream network had a demand driven diurnal daily flow for smaller diameter pipes in the distribution system and have pattern with an average downstream pressure of 42 m and a −1 started to be shown to be valid for large diameter transmis- typical daily demand profile varying between 6 and 20 L s −1 sion mains (Husband and Boxall, 2015). (0.14–0.48 m s ). The trunk main operates under gravity, As infrastructure ages, particulate material attached to the with a low point in the long section as shown in Fig. 1a. pipe wall can create a significant discolouration risk and re- duce hydraulic capacity (Cook and Boxall, 2011; Shahzad and James, 2002). To remove accumulated material, water 2.2 Fieldwork procedure and monitoring utilities often use invasive cleaning strategies as part of their rehabilitation programmes (AWWA, 2014). It is well estab- The trunk main was built around 40 years ago. No known lished that invasive cleaning (e.g. ice pigging, air scouring, cleaning interventions have been implemented since it was swabbing) can remove significant amounts of accumulated commissioned. As part of a scheme to reduce discolouration materials and biofilms from operational trunk mains (Ellison, customer contacts in the supplied DMAs and to remove par- 2003; Friedman et al., 2012). However, little is quantified re- ticulate material and biofilm attached to the pipe wall, pig- garding the efficacy of such interventions in terms of either ging with ice slurry was selected and implemented (Moore, quantity or quality improvement and how this changes over 2013; Quarini, 2002). The method uses ice slurry to form time following the intervention. a dynamic plug which increases shear on the pipe wall to The aim of this paper is to investigate the quantity and dislodge built-up material. A site-specific amount of semi- quality performance benefits of an in-service trunk main in- solid ice slurry was pumped through a UK standard washout vasive cleaning programme. Quantity performance of the hydrant at the trunk main inlet and emitted from a hydrant selected trunk main was assessed through hydraulic mod- 2.4 km downstream. Before cleaning, the main was isolated elling of monitored pre and post invasive cleaning flow and from the supply to ensure no mixing of water into the supply pressure data, with performance improvement evaluated by with the semi-solid ice pig. change in calibrated roughness height. Quality performance The trunk main served three separate downstream DMAs, was assessed by monitoring, modelling and comparing the with no connections over the 2.4 km length selected for turbidity response due to controlled increase in hydraulic cleaning. This layout ensures constant flow over the entire conditions pre and post intervention. Post cleaning eval- pipe length. Flow at the inlet, service reservoir level data and uations were repeated over a 12-month period to explore downstream pressure data allow for accurate hydraulic cali- longer-term performance changes. bration and quantity performance. Pre and post pressure data were collected using Syrinix Transientminder loggers with 15 min resolution. Flow was monitored continuously at the Drink. Water Eng. Sci., 10, 45–52, 2017 www.drink-water-eng-sci.net/10/45/2017/ I. Sunny et al.: Quantity and quality benefits of in-service invasive cleaning of trunk mains 47 Figure 2. Pre-cleaning intervention pipe cut out. service reservoir outlet using existing instrumentation, again around the full pipe circumference, supporting the PODDS at 15 min resolution. model concepts. The cut out was taken at the longitudinal Quality performance was assessed by imposing carefully low point, such that all gravitationally driven self-weight set- managed flow increases and monitoring for any turbidity re- tling processes that would have led to invert deposits were sponse; see Fig. 1b. From the PODDS concepts the imposed explored, with none being found. excess (above normal daily peak) shear stress would induce mobilisation of loose material associated with discoloura- 3.2 Hydraulic capacity assessment tion, with repeated operations of the same magnitude pro- viding insight into any inter-period accumulation or other Hydraulic models were developed and simulated in standard change