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...