Mass Balance

2.1. Introduction

The framework for the analysis of the origin and dynamics of particles is a mass balance model for a network. Within the model, the different processes dealing with particles can be quantified and analysed. The mass balance can be applied to the complete network, but can also be applied to selected parts of the networks.

The model of the mass balance is used to sum up the relevant processes involved in particle input and particle generation in the network. All these processes are interconnected and interdependent. Nevertheless for the sake of this study the processes will be separated as much as possible to simplify the analysis.

Next to the input and the generation of the particles, the kinetics of the particles' interaction are also important because they determine the fate of the particles in the network. Are they just transported through the network and supplied to the customers in low concentration or do they accumulate in the network adding to the mass content of the pipes? The balance between accumulated and mobile particles primarily determines the risk of discolouration.

Fig. 3 represents the mass balance schematically. The common discolouration event is a result of a high concentration of (coloured) particles in the water, disturbing the clarity to an extent that people notice it. Thus discolouration is the result of several processes in the water involving particles.

Suspended solids

Suspended solids

Suspended solids

Suspended solids

Biofilm Corrosion

Bed load transport

Fig. 3. Mass balance model

Bed load transport

Fig. 3. Mass balance model

The basic assumption is that accumulated sediment in combination with a hydraulic disturbance will lead to discoloured water. Consequently managing the transport and accumulation of the particles is the key to minimising of water quality deterioration.

The origin of the accumulated particles is multiple, as demonstrated in the above figure. To quantify the influence of the various processes this chapter is focussed on the concept of a mass balance model over the network or part of the network.

The basic elements of the mass balance model are described with respect to their contribution to the storage of particles. The storage of particles plays an important role in the generation of discolouration problems.

Storage = Mass in - Mass out + Mass produced

The term 'Mass in - Mass out' represents the transport phenomena in the pipes, while the term 'Mass produced' represents the particles generated as a result of biological or chemical processes such as biofilm growth, corrosion or coagulation, all of which produce mass and particles dependent on the quality of the bulk water. In formula:

CP*a*A*T + BP*/3*A*T + Q*T(BPP + CPP) (Mass produced) (1)

with

Q Volume flow

T Time scale used

Cp(in) Concentration particles in the incoming water in the form of suspended solids, particles etc

Cp(out) Concentration particles in the outgoing water in the form of suspended solids, particles etc Bed load out Transport of particles through bed load transport A Inner surface of the pipe a,i3 Factors applied to the active part of the surface giving corrosion products or biological parts respectively CP Corrosion production of particles from the pipe wall and released to the water

BP Biological production of particles from the biofilm and released to the water

BPP Biological particle production potential in the bulk fluid

CPP Coagulation particle production potential in the bulk fluid

'Mass in' and 'Mass out' are part of the water quality as represented by the suspended solids, particles etc. entering the control volume. These water quality parameters represent a direct input of mass into the control volume. Other parameters of the water quality also have an impact on the capability of production of mass. If aggressive water enters the control volume then interaction with cast iron will produce corrosion products. A high assimable organic carbon (AOC ) content will result in biofilm formation, but probably also in the formation of particles [1], 'Mass out' is measured in the same parameters as 'mass in' when looking at the water leaving the control volume. Mass will not only leave the control volume by the 'supernatant' water, but also through the bed load transport: sediment rolling on the bottom of the pipe. The 'mass storage' or AStorage is the net result of the mass balance. The behaviour of the storage is decisive for discolouration.

In the total analysis of discoloured water the role of the incoming mass is often neglected. Basically the incoming mass is the amount of suspended solids formed by the oxides of the elements Fe, Mn and A1 and in the Netherlands this can reach levels of up to 1.0 mg L"'of these oxides. Thresholds for Fe, Mn and A1 are respectively 200, 50 and 200 /xg L"1. As the elements are in the form of the oxides (Fe(OH)3, Mn02 and Al(OH)2) the actual mass load is 382, 79 and 578 /xg L"1. respectively. In total this is 1039 ¬°ig L"1. In normal practice this will be much lower as water rarely meets all the thresholds on these elements, still the amount of material can amount to a few hundred fig L"1. Other elements that contribute to the amount of incoming mass can be small sand or carbon particles from the filterbeds. Some other treatment processes can also contribute to the total suspended solids, for instance conditioning with marble filtration or pallet softening.

Suspended solids can also have organic characteristics. Gaulthier et al. [2], found in several samples an organic fraction of the suspended matter of 77%. Relating this to the inorganic load of up to 1.0 mg L"1 this would result in and organic load of up to 4,5 mg L"1 But again in practice this will rarely be reached. The contribution of suspended solids originating from the treatment plant is often neglected in literature. The water quality parameter dealing with particle load is turbidity. In Dutch legislation the maximum value is set on 1 NTU. The background for this boundary for turbidity is however more based on the disinfection efficiency than on particle load.

The suspended solids level is the amount of mass that actually enters the network and it has the potential to add to the sedimentary deposits in the pipe. Determining the suspended solids is not yet a routine measurement. As the quantities of the suspended solids will be very low, in the order of 50 to a few hundreds fig L"1, filtering of water over small mesh filters must be done with great care or with large quantities. The dilemma is to filter a relative small sample of water over a small mesh filter that would quickly clog and 'catch' as the suspended solids or to filter a larger quantity of water (200 to 300 litre) over a coarser filter that would not clog. The first method requires very sensitive equipment while the second one would be more robust.

2.2.1. Measuring mass in

The particle load directly from the treatment plant can best be evaluated using continuous monitoring techniques. At first, this will be the turbidity of the water and the particle counts and particle shapes. The turbidity or particle counts of the water leaving the treatment plant can show irregularities. Experience has shown that a large part of the sediment load to a network has its origin in these irregularities.

Particle counting at pumping station

Particle counting at pumping station

Mass Balance Pome Treatment

Time [hour]

Fig. 4. Particle counts at pumping station

Time [hour]

Fig. 4. Particle counts at pumping station

The particle counts of the treated water at a pumping station, shown in Fig. 4, shows a regular pattern of high counts, related to the backwash program. These irregularities are responsible for almost 75% of the total particle load to the system.

In addition to the continuous monitoring, samples can be taken from the incoming water and analysed for different components. Given that the irregularities in the treatment are largely responsible for the particle load, this makes the interpretation of the data obtained for grab samples, often taken at office hours, difficult, particularly in respect to any generally applicable conclusions.

Continuous monitoring of particles and turbidity can give an indication of the continuity of the treatment process and the associated particle or mass load to the system. The combination of the sample analyses of the dry solids and flow data enables the calculation of the total mass directly entering the system.

Samples of the water analysed for components such as Fe, Mn, dry solids, ATP etc. can be used to estimate the amount of mass that is introduced to the network. However these components are difficult to monitor on a continuous base. The relationship of these parameters with parameters that can be analysed on a continuous base such as turbidity and particle counts can help characterise the water quality and its ability to accumulate sediment.

2.3. Mass produced

Three different processes are recognised as being involved in the production of mass: corrosion of cast iron, biofilm formation and dying and coagulation of dissolved matter.

2.3.1. Corrosion of cast iron

There is a large amount of literature focussing on the relation between water quality and corrosion, and on the mechanism of corrosion as well as on the build up of corrosion layers [3, 4], However, little is known on the effect of corrosion on the particle content of the water and on the contribution to the amount of sediment. In the Netherlands corrosion velocities of approximately between 0.00 and 0.13 mm/yr were reported in the main sections in service areas of several treatment plants. A clear relation between the corrosion velocity and the corrosion index of the water however was not found. The objective of the study was to determine the decrease in the strength of a main due to cross sectional material loss. The weakest point in the surface is decisive for the residual strength of the pipe. The study demonstrated that corrosion is a local process resulting in pits and not in a global attack of the material. For an estimation of the contribution of corrosion to the particle load the corrosion surface is estimated to be 5 to 10% of the total surface. The iron (Fe) wall material will corrode with the formation of Fe203. The concentration of corrosion products in drinking water can be calculated by using the equation:

Cc Concentration of corrosion product (kg m-3)

vcor Corrosion velocity (m h-1 )

Ac Corroding wall surface (m2) (5 to 10% of total wall)

j0Fe Specific weight iron (7900 kg m-3)

a Element to oxide factor (Fe2C>3 => 1,43)

Q Discharge (m3 h"1)

Assuming a 1 km 100 mm diameter main with an average velocity of 0.1 m s"1 (2,8 m3 h"1) results in a mass load of 0.24 to 0.47 mg L"1 This is only a rough approximation since the actual mass load contribution depends on local conditions such as actual corrosion velocity, diameter of the cross section and the rate of discharge. In practice the results vary significantly and indicate that the above range of values is the maximum found in practice. However it does show that the contribution of corrosion to the particle load can be of the same order as that from the treatment plant, being up to a few hundred micrograms per liter. Considering the local nature of the corrosion and the stabilization of the process over years, the contribution of the treatment plant can very well be dominant.

2.3.2. Biofilm formation

Biological processes generate material from the AOC that is present in the water. Primarily this will materialise in the biofilm that is formed within the system. The biofilm formation potential can quantify the amount of biofilm that is formed in pipes. For further reading on this topic Kooij [1], is recommended. The equilibrium biofilm will be dynamic with a constant dying and regenerating of organisms. Within this dynamic equilibrium the biofilm will produce material that can add to the mobile part of the sedimentary deposits. Quantifying this part of the mass balance is still a challenge.

2.3.3. Coagulation of suspended and dissolved solids

In the finished water colloidal particles are present that can coagulate to larger particles that have finite settling rates. In some preliminary tests in the Netherlands, using a special developed flow-trough jar test, this phenomenon has been postulated, but it has not yet been proven [5], Research on the presence of aluminium in scales in water distribution systems showed a higher Al-content in scales in networks supplied with water treated with Al-based coagulants[6]. Future research is still required to substantiate the hypothesis of coagulation in the network.

2.4. Mass out

'Mass out' is separated into two different processes. Firstly, some material stays in suspension and leaves the control volume with the water. Secondly, there is the settled material that rolls along the bottom of the pipe in the form of a bed load transport. The several 'mass production' processes such as corrosion and biofilm formation and stability will contribute both to the suspended particles and to the settled particles.

By measurement of particle counts or turbidity as a general parameter for particles, the net result for storage can be evaluated. Fig. 5 shows the result of particle counts both at the pumping station and in the distribution network.

Bed load transport is very difficult to measure and although this is probably of overall significance in the transport of a large number of loose particles from the transportation lines to the reticulation system, it has yet to be accurately determined.

2.5. Storage

Storage is the net result of the mass balance. The mobility of the accumulated particles largely determines the discolouration risk. Disturbing the status quo will resuspend the particles and discolour the water to such a level that it becomes noticeable to the customer. Prevention of the resusupension is the key to better water quality in the network. In normal circumstances there is bound to be equilibrium between the 'mass in' and 'mass produced' with the 'mass out'. Disturbing this equilibrium, for instance through an increased velocity, will resuspend the sediment and cause discolouration. This phenomenon is also deliberately used to remove loose particles by flushing pipes with a high velocity surge. The net discolouration risk is consequently linked to both the amount of mobile sediment in the equilibrium and the likelihood of disturbances of the flow velocity.

Particle counting at pumping station and distribution network

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