J.H.G. Vreeburg

Simulating Nonresidential Water Demand with a Stochastic End-Use Model

A water demand end-use model was developed to predict water demand patterns with a small time scale (1 s) and small spatial scale (residence level). The end-use model is based on statistical information of users and end-uses: census data such as the number of people per household and their ages; the frequency of use; duration and flow per water-use event; occurrence over the day for different end-uses such as flushing the toilet, doing the laundry, washing hands, etc. With this approach, water demand patterns can be simulated. The simulation results were compared to measured water demand patterns on attributes such as peak flow and daily total water use, as well as on the shape of the pattern and the frequency distribution of flows and accelerations in flow. The simulation results show a good correspondence to measured water demands. Because the end-use model is based on statistical information rather than flow measurements, the model is transferable to diverse residential areas in different countries. Th...

Pyrosequencing reveals bacterial communities in unchlorinated drinking water distribution system: an integral study of bulk water, suspended solids, loose deposits, and pipe wall biofilm.

The current understanding of drinking water distribution system (DWDS) microbiology is limited to pipe wall biofilm and bulk water; the contributions of particle-associated bacteria (from suspended solids and loose deposits) have long been neglected. Analyzing the composition and correlation of bacterial communities from different phases helped us to locate where most of the bacteria are and understand the interactions among these phases. In the present study, the bacteria from four critical phases of an unchlorinated DWDS, including bulk water, pipe wall biofilm, suspended solids, and loose deposits, were quantified and identified by adenosine triphosphate analysis and pyrosequencing, respectively. The results showed that the bulk water bacteria (including the contribution of suspended solids) contributed less than 2% of the total bacteria. The bacteria associated with loose deposits and pipe wall biofilm that accumulated in the DWDS accounted for over 98% of the total bacteria, and the contributions of bacteria in loose deposits and pipe wall biofilm were comparable. Depending on the amount of loose deposits, its contribution can be 7-fold higher than the pipe wall biofilm. Pyrosequencing revealed relatively stable bacterial communities in bulk water, pipe wall biofilm, and suspended solids throughout the distribution system; however, the communities present in loose deposits were dependent on the amount of loose deposits locally. Bacteria within the phases of suspended solids, loose deposits, and pipe wall biofilm were similar in phylogenetic composition. The bulk water bacteria (dominated by Polaromonas spp.) were clearly different from the bacteria from the other three phases (dominated by Sphingomonas spp.). This study highlighted that the integral DWDS ecology should include contributions from all of the four phases, especially the bacteria harbored by loose deposits. The accumulation of loose deposits and the aging process create variable microenvironments inside loose deposits structures for bacteria to grow. Moreover, loose deposits protect the associated bacteria from disinfectants, and due to their mobility, the associated bacteria reach taps easily.

A fully adaptive forecasting model for short-term drinking water demand

For the optimal control of a water supply system, a short-term water demand forecast is necessary. We developed a model that forecasts the water demand for the next 48 h with 15-min time steps. The model uses measured water demands and static calendar data as single input. Based on this input, the model fully adaptively derives day factors and daily demand patterns for the seven days of the week, and for a configurable number of deviant day types. Although not using weather data as input, the model is able to identify occasional extra water demand in the evening during fair weather periods, and to adjust the forecast accordingly. The model was tested on datasets containing six years of water demand data in six different areas in the central and Southern part of Netherlands. The areas have all the same moderate weather conditions, and vary in size from very large (950,000 inhabitants) to small (2400 inhabitants). The mean absolute percentage error (MAPE) for the 24-h forecasts varied between 1.44 and 5.12%, and for the 15-min time step forecasts between 3.35 and 10.44%. The model is easy to implement, fully adaptive and accurate, which makes it suitable for application in real time control.

Flexible design in water and wastewater engineering--definitions, literature and decision guide.

Urban water and wastewater systems face uncertain developments including technological progress, climate change and urban development. To ensure the sustainability of these systems under dynamic conditions it has been proposed that technologies and infrastructure should be flexible, adaptive and robust. However, in literature it is often unclear what these technologies and infrastructure are. Furthermore, the terms flexible, adaptive and robust are often used interchangeably, despite important differences. In this paper we will i) define the terminology, ii) provide an overview of the status of flexible infrastructure design alternatives for water and wastewater networks and treatment, and iii) develop guidelines for the selection of flexible design alternatives. Results indicate that, with the exception of Net Present Valuation methods, there is little research available on the design and evaluation of technologies that can enable flexibility. Flexible design alternatives reviewed include robust design, phased design, modular design, modular/component platform design and design for remanufacturing. As developments in the water sector are driven by slow variables (climate change, urban development), rather than market forces, it is suggested that phased design or component platform designs are suitable for responding to change, while robust design is an option when operations face highly dynamic variability.

Radial transport processes as a precursor to particle deposition in drinking water distribution systems

Various particle transport mechanisms play a role in the build-up of discoloration potential in drinking water distribution networks. In order to enhance our understanding of and ability to predict this build-up, it is essential to recognize and understand their role. Gravitational settling with drag has primarily been considered in this context. However, since flow in water distribution pipes is nearly always in the turbulent regime, turbulent processes should be considered also. In addition to these, single particle effects and forces may affect radial particle transport. In this work, we present an application of a previously published turbulent particle deposition theory to conditions relevant for drinking water distribution systems. We predict quantitatively under which conditions turbophoresis, including the virtual mass effect, the Saffman lift force, and the Magnus force may contribute significantly to sediment transport in radial direction and compare these results to experimental observations. The contribution of turbophoresis is mostly limited to large particles (>50 μm) in transport mains, and not expected to play a major role in distribution mains. The Saffman lift force may enhance this process to some degree. The Magnus force is not expected to play any significant role in drinking water distribution systems.

COMBINING THE PROBABILISTIC DEMAND MODEL SIMDEUM WITH A NETWORK MODEL

Modeling of water quality in water distribution systems can be improved significantly when using stochastic demands. The stochastic demand pattern generator SIMDEUM is based on stochastic information on end users as retrieved from surveys. Stochastic demand patterns from SIMDEUM were applied in a small network model of 550 demand nodes in a residential area in the Netherlands. An EPANET based network solver was used to assess velocities and travel times. The actual measurements agree well with the simulations.

THE SELF-CLEANING VELOCITY IN PRACTICE

The Dutch drinking water companies are constructing velocity based self-cleaning residential drinking water distribution systems (DWDS). Field studies with particle counters have shown that these DWDS indeed do not foul. Laboratory studies have shown the settlement and re-suspension of particles in water mains under constant flow conditions. However, the relation between mains fouling and hydraulic conditions under realistic (variable) flows has not been determined. In the presented study, the effect of variable flow velocities on particles in a real residential DWDS was studied through a detailed analysis of turbidity measurements during flushing in combination with a detailed EPANET network model. Firstly, each pipe stretch was flushed with 1.5 m/s for three turnovers and most of the pipes appeared to be clean after the first turnover. This means that it was possible to link the measured turbidity to the location in the pipe from where it was re-suspended. Secondly, an all-pipes EPANET network model was filled with realistic demand patterns from the end-use model SIMDEUM; a small hydraulic time step (0.01 h) was used. This allowed for determining the maximum daily velocity occurring in each pipe stretch. The combination of the first and second step led to a graph of resuspended turbidity against maximum daily velocity. The study showed that in this residential DWDS there is a threshold value for the maximum velocity of 0.2 to 0.25 m/s above which the pipes remain clean. Thus, the existence of the self-cleaning velocity was demonstrated. This study helps in understanding particle behavior in DWDS.

COMPARISON OF WATER DEMAND MODELS: PRP AND SIMDEUM APPLIED TO MILFORD, OHIO, DATA

There is growing interest in modeling water demands on short time scales (as brief as one second) and small spatial scales (typically single homes). Buchberger et al. (1996, 2003) have developed the Poisson Rectangular Pulse (PRP) model for this purpose. Blokker et al. (in prep.) have developed an end-use model SIMDEUM (which stands for SIMulation of Demand, and End-Use Model) which is based on statistical information from end-uses and does not require any flow measurements. SIMDEUM was developed and validated for Dutch water use. In this paper the PRP model and SIMDEUM are compared with each other and with measured indoor water demands from 21 homes in Milford, Ohio. Both models compare well to the measurements; the PRP model works better in simulating the cumulative flows of a sum of 20, SIMDEUM works better in simulating the flows of a single home.

Centralised, decentralised or hybrid sanitation systems? Economic evaluation under urban development uncertainty and phased expansion

Sanitation systems are built to be robust, that is, they are dimensioned to cope with population growth and other variability that occurs throughout their lifetime. It was recently shown that building sanitation systems in phases is more cost effective than one robust design. This phasing can take place by building small autonomous decentralised units that operate closer to the actual demand. Research has shown that variability and uncertainty in urban development does affect the cost effectiveness of this approach. Previous studies do not, however, consider the entire sanitation system from collection to treatment. The aim of this study is to assess the economic performance of three sanitation systems with different scales and systems characteristics under a variety of urban development pathways. Three systems are studied: (I) a centralised conventional activated sludge treatment, (II) a community on site source separation grey water and black water treatment and (III) a hybrid with grey water treatment at neighbourhood scale and black water treatment off site. A modelling approach is taken that combines a simulation of greenfield urban growth, a model of the wastewater collection and treatment infrastructure design properties and a model that translates design parameters into discounted asset lifetime costs. Monte Carlo simulations are used to evaluate the economic performance under uncertain development trends. Results show that the conventional system outperforms both of the other systems when total discounted lifetime costs are assessed, because it benefits from economies of scale. However, when population growth is lower than expected, the source-separated system is more cost effective, because of reduced idle capacity. The hybrid system is not competitive under any circumstance due to the costly double piping and treatment.

Sizes for Self-Cleaning Pipes in Municipal Water Supply Systems

Historically, the minimum diameter of a pipe in a municipal water distribution system has been governed by fire flow requirements. In most cases across North America, the minimum allowable pipe size is six inches (150 mm), even on branching stubs that feed 100 or fewer homes. These peripheral regions of the pipe network often experience low velocities, long residence times, poor water quality and accumulation of settled deposits. As a consequence, many water utilities routinely flush these lines in an effort to cleanse them. In recent years, several water utilities (most notably in the Netherlands) have introduced the concept of a “self-cleaning” network where, for several minutes each day, a critical pipe velocity occurs and resuspends particles that settled during the low flow periods. Self-cleaning networks are a radical departure from the typical looped distribution system. Self-cleaning networks have a branch-type arrangement with downstream declining diameters and unidirectional velocities that reach at least 0.4 m/s for several minutes each day. Peak flows in the self-cleaning network are estimated using an empirical “ qN ” method, where q is the tapping unit demand usually taken as 0.083 litre per second per unit and N is the number of tapping units in the neighborhood. The problem of estimating the peak flow and, hence, the required pipe diameter can also be investigated using basic principles from the Poisson Rectangular Pulse (PRP) model for residential water demands. The objective of this paper is to apply theoretical results from the PRP model to generate reliability based estimates of the peak flow and the corresponding pipe size needed to assure a critical self-cleaning velocity as a function of neighborhood size. Results from the PRP model are compared against the conventional but conservative qN method.