J.R. West

THE POTENTIAL FOR BIOFILM GROWTH IN WATER DISTRIBUTION SYSTEMS

Biofilms on pipe walls in water distribution systems are composed of bacteria in a polymeric matrix, which can lead to chlorine demand, coliform growth, pipe corrosion and water taste and odour problems. The majority of previous studies have been laboratory or pilot plant based and few results are available for field conditions. In this study, field observations of biofilm were made using biofilm potential monitors. The monitor results were compared with pipe samples taken from the distribution system and with laboratory pipe reactors. An empirical equation quantified the inhibitory effects of free chlorine and decrease of temperature on biofilm growth. With water having total organic carbon concentrations in the range 1.5-3.9mg/1 a free chlorine residual of 0.2 mg/l was needed to reduce biofilm concentration to below 50 pg ATP cm2. Pipe material influenced biofilm activity far less than chlorine with mean biofilm activity being ranked in the order glass (136 pg ATP/cm2) < cement (212 pg ATP/cm2) < MDPE (302 pg ATP/ cm2) < PVC (509 pg ATP/cm2).

Factors which control bulk chlorine decay rates

Abstract Several computer programs for modelling water distribution networks have been developed which incorporate a facility for modelling chlorine decay. Problems have been experienced with the calibration and durability of these models due to both temporal and spatial variability in the decay constants. Chlorine will decay either due to reactions at the pipe wall or due to reactions in the bulk water. The aim of the work presented in this paper is to investigate the factors which influence bulk decay. Over 200 determinations of bulk chlorine decay against time were performed on waters taken from 32 sampling locations within the Severn Trent region, U.K. The bulk decay constant was observed to show significant variation with temperature, the initial chlorine concentration and the organic content of the water. An equation was derived relating these parameters which could be used to update the decay constants in network models and improve their durability.

The decay of chlorine associated with the pipe wall in water distribution systems

Free chlorine decay rates in water distribution systems for bulk and wall demands should be modelled separately as they have different functional dependencies. Few good quality determinations of in situ wall demand have been made due to the difficulty of monitoring live systems and due to their complexity. Wall demands have been calculated from field measurements at 11 locations in a distribution system fed from a single source. A methodology for the laboratory determination has been evolved and shown to give results that are similar to the in situ results. Pipe materials were classified as either having high reactivity (unlined iron mains) or low reactivity (PVC, MDPE and cement-lined ductile iron). The results indicate that wall decay rates for the former are limited by chlorine transport and for the latter by pipe material characteristics. The wall decay rate is inversely related to initial chlorine concentration for low reactivity pipes. In general, water velocity increases wall decay rates though the statistical confidence is low for low reactivity pipes. A moderate biofilm coating did not influence the wall decay rate for low reactivity pipes.

Use of continuous turbidity sensor in the prediction of fine sediment transport in the turbidity maximum of the Trent Estuary, UK

Results from continuous monitoring of turbidity and water level at Burringham, on the tidal River Trent, UK, are presented for the period 18 May 1997 to 9 February 1998. These measurements, together with detailed readings of velocity and suspended sediment concentration over an individual tidal cycle near the opposite bank at Derrythorpe, help to describe the mechanisms and behaviour of the turbidity maximum (TM). It is demonstrated that there is a distinct pattern of fine sediment movement that reflects a predictable system response to changing hydraulic features. It is shown that the TM in this system is highly mobile, and its location depends on antecedent fresh water flow, and tidal range. Approximate representative flood and ebb tide suspended sediment concentrations of up to 13 g/l over this nine-month period have been derived from the data and plotted against fresh water flow and tidal range, in order to show the relationship between these parameters. Three semi-empirical polynomial regression models have been tested for goodness of fit against available data. It was found that a partitioning approach, whereby data are grouped into different categories depending on antecedent fresh water flow, yielded the lowest standard error for the period analysed. Analysis of detailed observations of suspended sediment concentration and velocity measured over an individual tidal cycle also help to elucidate the mechanism of tidal pumping within this system. These results also help to give an estimate of the relative magnitude of suspended sediment fluxes during typical low fresh water flow conditions. It is estimated that for low fresh water flow conditions, a typical spring tide can mobilise at least an order of magnitude more sediment than a neap tide.

Evaluation of different analytical methods for tracer studies in aeration lanes of activated sludge plants

Abstract There are many different methods available for analysing the results of tracer studies on the aeration lanes of activated sludge plants. If the results are to be used for modelling then it is necessary to calculate the number of tanks in series to allow the dispersion within the tanks to be accurately modelled. Two methods for this are presented in this paper and both are based on Levenspiels work. The WRc method calculates the number of tanks in series but provides information on short-circuiting only by visual inspection of the residence time distribution (RTD) curve. The Martin method produces information on mean residence time and number of tanks in series for a number of separate flow “strands” through the tank. In this way, “short-circuiting” can be parameterised in a manner consistent with mathematical models of the activated sludge process. Other methods for analysing RTD curves, whilst giving useful information on the flow regime within the tanks, are not suitable for use in dynamic models of wastewater treatment.

Measuring sediment exchange rates on an intertidal bank at Blacktoft, Humber Estuary, UK

Results from a suite of Photo Electronic Erosion Pins (PEEPs) and manual pins installed on an intertidal bank at Blacktoft, near the confluence of the Rivers Trent and (Yorkshire) Ouse, UK are presented for summer 1997 (1 May-28 September). These reveal a pattern of erosion and deposition, which can be related to variations in tidal range, freshwater flow and wind speed over the period. During spring tides, greater resuspension of bed sediment leads to a greater availability of sediment in the water column for deposition on the bank. High wind speeds cause greater erosion of material from the bank due to wind-induced wave action. These processes of sediment exchange are also modified by the effects of biological activity on the sediment and of consolidation. It was demonstrated that the mean daily change in elevation of the upper part of the bank at Blacktoft was approximately 11 mm, which is two-three-fold less than the equivalent figure measured by a similar method at Burringham on the River Trent. It is thought that this difference is due to the effects of a greater concentration of suspended sediment settling onto the banks at Burringham, which are also subject to greater erosion due to their steeper slope. Results from a longer and more widespread survey of eight other intertidal banks in the Trent-Ouse Estuary system suggest that deposition and erosion occur in phase on all intertidal banks within the study area. Intertidal banks towards the upstream end of the system show much less variation in bank level than those further downstream.

The determination and prediction of longitudinal dispersion coefficients in a narrow, shallow estuary

Abstract Measurements of velocity and salinity for parts of a flood and an ebb tide in the Conwy estuary have been used to evaluate the time-dependent longitudinal dispersion coefficients due to vertical and transverse oscillatory shear effects. The values have been compared with semi-analytical predictive formulae. Simple empirical expressions are suggested for relating the dispersion coefficients to hydraulic parameters for the conditions observed.

Turbulent perturbations of velocity in the conwy estuary

Abstract Field measurements of the vertical and horizontal components of fluid velocity 0·43 m above the bed have been made with an electromagnetic flowmeter in a partially mixed reach of the Conwy estuary for parts of a flood and an ebb tide. The turbulent mean velocity and density fields showed different effects for flood and ebb tides caused by the interaction of shear and the longitudinal density gradient. The turbulent velocity parameters were generally dominated by bed-generated turbulence effects, but significant longer period contributions attributed to the shear-density interactions were detected. The shear-stress measurements made by two independent methods showed good agreement. The results generally show good agreement with previous less complete data. The use of the Richardson number to quantify vertical stability needs further consideration for partially mixed estuarine flows.

Vertical turbulent mixing processes on ebb tides in partially mixed estuaries

Abstract Measurements of turbulent fluctuations of the horizontal and vertical components of velocity and of salinity in the region 1-1·25 m above the bed have been made in the Teign estuary during parts of three ebb tides of different tidal ranges. The turbulent mean flow field determined from vertical profiles of velocity and salinity varied from well mixed to partially mixed depending on the relative importance of the vertical gradient promoting effect of shear and longitudinal density gradient and of the mixing effect of bed-generated turbulence. The turbulence parameters showed regular temporal trends. The relative intensity values for the new and existing data for the velocity components are strongly dependent on relative depth but not generally affected by Richardson number. The intensity of the salinity fluctuations increased with the salinity gradients. The vertical transport of momentum and the vertical Reynolds flux correlation coefficients decreased with Richardson number. The horizontal Reynolds flux correlation coefficient increased with Richardson number. These effects are tentatively explained by the concept of turbulent fluctuations being replaced by wavelike fluctuations in stable conditions. Results for momentum and solute mixing lengths help to substantiate previously published values.

A note on flow structure in the Great Ouse estuary

Abstract Field measurements of vertical profiles of velocity and salinity along with turbulence measurements have been used to examine the effect of density gradients on the flow structure in the Great Ouse estuary. During the flood tide shear and longitudinal density gradients cause well mixed conditions in the lower part of the water column. In the upper part of the water column secondary flow effects induced by transverse density gradients, and acceleration effects can contribute to the formation of stable vertical density gradients. On the ebb tide the vertical density gradient appears to be the dominant factor which determines the structure of the flow. The velocity and shear stress data show the evidence of large scale motions which are consistent with the postulated flood and ebb flow structures.