Introduction

An awareness of the problems caused by solids in sewer systems has been apparent since the first cloacae (sewer) system was built in Rome in the 6th century BC. However, it was only with the advent of industrial society and the increasing concentration of humanity into cities, that the problems became acute (Halliday, 1999). The 18th century saw a number of developments that gave a compelling impetus to the construction in cities of 'designed' underground tunnels (sewers) for the collection of sewage (Markham, 1994); for example, utilisation of the WC (generally acknowledged as invented in 1589, although there is earlier evidence for some form of water closet) and linkage of cholera transmission directly to inadequate sanitation facilities in London by Dr John Snow in the 1850s. In the year 1854 alone, some 1000 miles of new sewer pipes were laid in towns in Britain. At this time a number of innovations were introduced to attempt to ensure that the solids collected in the drains and sewers of London were 'conveyed most cheaply and innoxiously to any distance out of towns' (Chadwick, 1842). These included the invention of egg-shaped sewers and the use of in-sewer flushing systems (Bertrand-Krajewski, 2002). Some engineers, however, refused to acknowledge that smaller sewers, by giving the best means of transporting the solids, were preferable to larger, and maintained that larger entry sizes were essential for maintenance. Others propounded the use of flat-bottomed sewers and eschewed the use of 'circular pipes', as it was thought that the shape of the cross section was not particularly important for sediment transport (Binnie, 1981). Such disagreements are still not entirely resolved even today, as discussed in Sections 2.3 and 4.2 of this report. It was also realised at this time that the nature and operation of sewerage systems was inseparably linked to the supply of water, as intermittency in supply resulted in more solids problems; hence the view that water and wastewater systems had to be considered holistically was recognised as early as the 19th century. Figure 1.1 illustrates a brick culvert more than 150 years old still in use in Dundee until the 1990s. Figure 1.2 illustrates, more typically, that many of the early sewers and those constructed more recently, differ little.

Since these early developments, advances in sewerage and methods for managing the solids have been more modest. While the fabric has been made stronger, new materials such as vitrified clay and concrete have been utilised and the use of flexible joints has become widespread, system performance understanding has advanced mostly in terms of the hydrology and hydraulics of the inputs, and the hydraulics of the flows in the system.

Knowledge about behaviour of the solids and the best means of managing them remained largely as it was at the end of the 19th century until very recently. It is true that the problems caused by in-sewer solids are often unacknowledged, even today, and differentiated investment policies which favour capital rather than operational expenditure discriminate against a realistic look at these problems, leading to many controlling authorities and utilities preferring not to be aware of in-sewer solids problems.

The use of computer-based hydrologic and hydraulic deterministic simulation models for the above- and below-ground drainage processes is now ubiquitous, and the first generation of models linking flows to solids and pollutants being transported has emerged. Nonetheless many questions remain, and surprisingly it was only within the past 20 years that concerted research was initiated in a number of European countries to investigate the origins, movement, nature and effects of the solids entering, moving and discharged from sewerage systems. These initiatives, together with studies in the USA and Japan, were primarily undertaken to determine and control the discharges from combined sewer overflows, and were driven by aspirations to develop predictive computer models.

Sewer solids comprise a wide variety of very small (sub-micron) to large (centimetre) particles, which may be classified in terms of their physical, chemical and biological characteristics; they originate from a variety of sources as shown diagrammatically in Figure 1.3. It is important to appreciate that very small microorganisms, which are of major significance in wastewater systems, are a part of this overall group of 'sewer solids', as illustrated in Figure 1.4 (adapted from Sections 2.2.1 and 2.2.2 of this report).

Early studies in the 1970-1980s (Lindholm and Balmer, 1978; Dauber and Novak, 1982; Krejci et al., 1987) showed that sediments in sewers were easily resuspended during wet weather flows in combined sewers, contributing significantly to overflow spill loads of solids and pollutants. Later studies continue to confirm the importance of sediments both for efficient sewer operation (Crabtree, 1989) and receiving watercourse impact (Rees and White, 1993).

From the continuing work a number of important results have emerged. In the UK, for example, it was concluded in 1986 that the sediment deposits in the nation's sewers were costing some £60 million per annum to manage (CIRIA, 1986). Sewer solids have been shown to be reservoirs of pollutants, often readily available for re-release by even fairly small increases in flow (Gromaire et al., 2001).

Many current studies are concerned with minimising the effects from these 'eroding events', or 'flushes', on receiving water bodies; new ideas have emerged for better sewer design and operation, as well as for the design and operation of storage systems both in-sewer and at the ends of pipes (e.g. Pisano et al., 2003).

Notwithstanding these new initiatives, there are many areas of uncertainty related to sewer solids; as well as influencing thinking about system operation, these uncertainties affect the way in which concepts of 'sustainability' may be introduced. In industrialised countries the possibility of a 'sewerless' society appears to be very remote, despite its apparent potential to be more sustainable in many cases.

Areas where research which includes aspects of sewer solids is underway or is needed are outlined in Table 1.1.

This Scientific and Technical Report presents a review of knowledge relating to solids and sewer systems. It is written to be accessible to a broad audience, not only those responsible for wastewater systems. Chapter 2 deals with sewer solids and associated pollutants: the origins (Section 2.1), the characteristics and transformative processes related to sewer solids (Section 2.2) and the movement, transport, deposition and erosion of the solids (Section 2.3). Chapter 3 considers the effects of sewer solids. The effects are introduced in Section 3.1, with the conveyance and potential blockage problems considered in Section 3.2. Section 3.3 considers the formation of sulphides as related to sewer solids and the effects these may cause. The potential contribution of solids to flushes and wet weather pollution are considered in Section 3.4; Section 3.5 considers the effects of sewer solids on the performance of ancillary structures and wastewater treatment facilities. Other effects such as those caused by fats and greases are considered in Section 3.6. Chapter 4 deals with the management of sewer systems to minimise the occurrence and impacts caused by solids. Section 4.1 deals with controls at source and input options, Section 4.2 considers sewer design, operation and maintenance and Section 4.3, the control of solids in tanks, CSO and in settlement devices. Chapter 5 concludes, briefly reviewing new ideas and future directions in relation to solids in sewers.

Sewer solids characteristics and processes

2.1 ORIGINS

2.1.1 Introduction

Solids entering sewer systems originate from a variety of sources. Five principal sources are typically defined:

• the atmosphere, which contains fine dust and aerosols;

• the surfaces of the catchment, where solids accumulate during dry weather periods and are washed off during storm events: roofs, streets, parkings, highways, etc.;

• domestic sewage, which contains the largest proportion of organic solids;

• the environment and processes inside the drainage/sewer system: natural water body interactions, infiltration/exfiltration, decay and degradation of solids;

• industrial and commercial effluents and solids from construction sites, which typically may contribute very significantly to the solids loads entering sewers.

In the following sections, the characteristics and processes relative to each source of solids will be briefly considered.

2.1.2 The atmosphere

The atmosphere contains fine dust particles and aerosols which contribute to raindrop formation. These particles originate from different sources which can sometimes be remote from the catchment where they will precipitate: heating, automobile traffic, waste incineration, industry, construction sites, erosion of natural soils, etc. During storm events, these particles are transported within raindrops. As they reach the ground at the very beginning of the event, the pollutant load is usually independent of the duration and of the intensity of the rainfall (Randall et al., 1978; Göttle, 1978a, 1978b).

The contribution of these particles to the total mass of solids during storm events is usually less than 10 %, and the suspended solids concentration in rainfall water ranges from 1 to 10 mg/l, with mean values of about 3-4 mg/l (Göttle, 1978a, 1978b; Novotny et al., 1985; Deutsch and Grange, 1986; Artieres, 1987, Uchimura et al., 1996). Göttle (1978a, 1978b) and Ellis (1986) proposed a simple relationship giving the mass of solids transported by raindrops:

Me = 1 -(1 - ed • Dng) (2.1.1)

where Me is mass of solids transported by rainfall (expressed as a percentage of available mass in the atmosphere)

ed is efficiency of the collision between solids of diameter d and raindrops of

diameter Dg (ed is correlated with rainfall intensity and duration)

n is number of raindrops per unit surface.

If the mass of atmospheric solids transported by raindrops is not very significant in comparison with other sources, the contribution to the pollutant load can be high or even very high for some pollutants associated with the finest particles, such as PAHs (polycyclic aromatic hydrocarbons) and PCBs (polychlorobyphenyls) (Bertrand-Krajewski, 1993).

Many measurements of pollutant concentrations in rainwater have been undertaken. General ranges for COD (chemical oxygen demand), heavy metals and hydrocarbon mean concentrations are summarised in Table 2.1.1. The broad ranges are due to the significant variability of local conditions. A significant proportion of the pollutants is associated with the particulate fractions (Section 2.1.3.2.8).

2.1.3 Catchment surfaces

Solids washed off from the different surfaces of a catchment are the main contribution to the pollutant load of runoff water. For practical reasons, four primary sources of 'surface solids' are usually distinguished:

• roofs

• streets, highways and car parks

• gullies

• de-icing.

In addition, permeable and other 'natural' surfaces also contribute, especially during heavy rainfall events.

A recent study by Armitage and Roseboom (2000) reviewed a number of reports about urban litter that finds its way into watercourses. It was concluded (p. 183) that 'there was an infinite variety of the types and quantities of litter washed off a catchment'. In a South African study, in a catchment of 299 ha (85 % commercial/industrial), of some 1,467 m(139 365 kg) of litter that found its way onto streets per year, some 18 % (25 000 kg) was eventually found in the storm drainage system. The litter was reported to be transported in the open channel drainage system with distinct foul flushes following long dry periods. The first flush of the wet season was comprised of dense solids: bricks, stones, tyres, down to fine sand, together with suspended debris which was mostly plastic bags. Animal carcasses and a lot of floating material were also present.

2.1.3.1 Roofs

The solids washed off from roofs during storm events have a number of different origins:

• solids from atmosphere deposited during dry weather periods;

• solids from chemical and physical degradation of the roof and gutter material itself;

• solids from bird droppings, branches and leaves.

Solids from the atmosphere result mainly from human activities: heating, automobile traffic, waste incineration, industries, etc., as described in Section 2.1.2. These solids are very fine and their median diameter is lower than 30 pm (Novotny et al., 1985). Depending on meteorological and local conditions, they can be transported by winds over long distances, and their deposition rates are highly variable. Some assessment of dry weather period deposition rates have been carried out, and results are presented in Table 2.1.2. As explained above, the wide ranges are due to the variability of local conditions.

Illustrative solids concentrations in roof runoff are presented in Table 2.1.3. The ranges are broad as the concentrations depend on the site, rainfall intensity, and nature, shape and state of the roof.

Ellis (1986) concluded that the solids in roof water constitute about 15 to 30 % of the total mass of solids in runoff water during storm events. Roof water also contains other pollutants, which are frequently mainly associated with the particulate phase (Forster, 1996). Some ranges of concentrations for heavy metals and organic compounds are given in Table 2.1.4.

In addition to the direct deposition of particles onto roofs, vapour phase pollutants can also be adsorbed and absorbed by falling particles. For thermodynamical reasons, these pollutant gases in the atmosphere do not remain in this phase but interact with water molecules and dissolve in small water particles, resulting in high gas concentrations. These water particles also interact with aerosols, which are defined as a combination of solid and concentrated aqueous phases produced from various sources, particularly combustion and photochemical pollution (Hough, 1988).

Complex models combining fundamental aspects of the physics, chemistry and thermodynamics of gases and small atmospheric water particles have been proposed to describe the atmospheric fallout of aerosols and gases (see, for example, Hough, 1988; Chamberlain, 1986). However, these sophisticated models have not been widely used in the context of urban hydrology where 'lumped' or aggregated approaches are preferred.

2.1.3.2 Streets

2.1.3.2.1 Introduction

Streets, highways and parking areas constitute the most common impervious surfaces in urban catchments, and are the main source of solids transported, via runoff, into the sewer system. The solids originate from:

• erosion of road material;

• wear of vehicles tyres;

• dry deposition of fine solids from the atmosphere;

• solid wastes and litter (cans, broken glass, bottles, pull tabs, papers, building materials, plastic, garbage, etc.) which can also be classified as 'gross solids';

• animal droppings and vegetation;

• de-icing materials;

• construction sites.

In addition, in coastal areas, wind blown sand may arise from all types of surface.

A study by Shaheen (1975) in the Washington, D.C. Metropolitan area quantified the traffic contributions. This study estimated that approximately 0.7 g/axle.km of roadway of solids can be directly attributed to traffic. In another US Environmental Protection Agency (USEPA) study, direct traffic emissions were reported to be 0.2 g/vehicle.km from vehicle exhaust and 0.125 g/vehicle.km from tyre wear (EPA, 1977). Heavy traffic also degrades road surfaces and increases solids loads.

In residential areas, fallen leaves and vegetation residues, including grass clippings, dominate street refuse composition during the autumn. A mature tree can produce from 15 to 25 kg of organic leaf residue (dry weight) that contains significant amounts of nutrients (Heaney and Huber, 1973). A typical value of foliage and leaf fallout for a forested area in Minnesota with some 420 trees/ha is about 3800 kg/ha.year or approximately 9 kg/tree. Of the yearly values, about 65 % of the fallout occurs during the autumn. The fallen leaves are about 90 % organic and contain about 0.04 to 0.28 % of phosphorus. Figure 2.1.1. shows the important elements of solids transport and accumulation on street surfaces.

2.1.3.2.2 Solids location

The solids are not uniformly distributed on street surfaces. For a typical street, three regions may be distinguished: the road, the gutter and the footpath. More than 80-85 % of the solids are accumulated in the first metre from the footpath (Sartor et al., 1974). This result has been confirmed by many researchers (e.g. Artieres, 1987; Novotny et al., 1985; Leduc and Ouldali, 1989). According to Artieres (1987):

• 50 % of the total mass is located in the gutter;

• 90 % of particles less than 80 pm in size are in the gutter, with only a few fine particles on the road and on the footpath.