Wastewater Chapter 3 PDF

Chapter 3 Wastewater 3.1 INTRODUCTION Wastewater, or sewage, is one of the two major urban water-based flows that form the basis of concern for the drainage engineer. The other, stormwater, is described in Chapter 5. Wastewater is the main liquid waste of the community. Safe and efficient drainag...

Chapter 3 Wastewater 3.1 INTRODUCTION Wastewater, or sewage, is one of the two major urban water-based flows that form the basis of concern for the drainage engineer. The other, stormwater, is described in Chapter 5. Wastewater is the main liquid waste of the community. Safe and efficient drainage of wastewa- ter is particularly important to maintain public health (because of the high levels of potentially disease-forming micro-organisms in wastewater) and to protect the receiving water environ- ment (due to large amounts of oxygen-consuming organic material and other pollutants in wastewater). This chapter provides background information and summary data on wastewa- ter. The quantification for design purposes is dealt with in Chapter 8. The basic sources of wastewater are summarised in Figure 3.1 and consist of Domestic Non-domestic (commercial and industrial) Infiltration/inflow In practice, the relative importance of the components varies with a number of factors, including Location (climatic conditions, the availability of water and its characteristics, and individual domestic water consumption) Diet of the population Presence of industrial and trade effluents Type of collection system (i.e., separate or combined) Condition of the collection system This chapter is concerned with the generation and characteristics of wastewater. It col- lates quantity and quality information on the various sources of wastewater and discusses their relative importance. Section 3.2 covers domestic sources of wastewater, Section 3.3 non-domestic, and Section 3.4 the contribution of infiltration and inflow. Wastewater qual- ity issues are dealt with in Section 3.5. 3.2 DOMESTIC In many (but not all) networks, the domestic component of wastewater is the most impor- tant. Domestic wastewater is generated primarily from residential properties but also includes contributions from institutions (e.g., schools, hospitals) and recreational facilities 43 44 Urban Drainage Water supply Rainfall Wastewater Domestic Commercial Industrial Infiltration Inflow Figure 3.1 Sources of wastewater. (such as leisure centres). In terms of flow quantity, the defining variable is domestic water consumption, which is linked to human behaviour and habits. In fact, very little water is actually consumed, or lost from the system. Instead, it is used intermittently (degrading its quality) and then discharged as wastewater. Hence, in this section we look at the links between water usage and wastewater discharge and, in particular, how these vary with time. 3.2.1 Water use Important factors affecting the magnitude of per capita water demand include the following. 3.2.1.1 Climate Climatic effects such as temperature and rainfall can significantly affect water demand. Water use tends to be greatest when it is hot and dry, due largely to increased garden watering/sprinkling and landscape irrigation. The impact on wastewater is less pronounced, as this additional water will probably not find its way into the sewer. 3.2.1.2 Demography It has been demonstrated that household occupancy levels are important, with larger families tending to have lower per capita demand (Willis et al., 2013). While, at the other end of the scale, retired people have been shown to use more water than the rest of the population (Russac et al., 1991). 3.2.1.3 Socio-economic factors The greater the affluence or economic capabilities of a community, the greater the water use tends to be. Work in the UK (Russac et al., 1991) and Australia (Willis et al., 2013) has confirmed the link between water demand and economic indicators such as dwelling type, dwelling rateable value, and household income. This is probably due to greater ownership and use of water-using domestic appliances such as washing machines, dishwashers, and power showers. 3.2.1.4 Development type Dwelling type is important. In particular, dwellings with gardens may use more water than flats or apartments.  Wastewater 45 3.2.1.5 Extent of metering and water conservation measures Water undertakers with metered supplies usually charge their customers based on the quan- tity of water used in a given period. Systems with unmetered services charge a flat rate for unlimited water use. In theory at least, metered supplies should prevent waste of water by users, reduce actual water use, and therefore reduce wastewater flows. Water is not uni- versally metered in the UK with only about 50% of houses metered, albeit with a rising trend and wide regional variations (CCW, 2016). It has been shown by Ornaghi and Tonin (2015) in their study of the widespread rollout of meters in Southeast England (January 2011–September 2014) to result in a water saving of 16.5%. Variable tariffs allied to smart meters could increase these savings further. Water conservation measures such as low-flow taps/showers, low-flush toilets, and recycling/reuse systems also reduce water demand. The potential impacts on the urban drainage system of more widespread use of such measures are discussed further in Chapters 9 and 24. 3.2.1.6 Quantification Water consumption per head of population can be extremely varied, as shown in Figure 3.2. However, average domestic water usage in England and Wales stood at 140 L/hd.d in 2015/2016 (CCW, 2016). Usage ranges from 95 to 224 L/hd.d in the 11 countries cited by Friedler et al. (2013). Water is used in three main areas in the home. Approximately one third of the water is used for WC flushing; one third for personal washing via the wash basin, bath, and shower; and the final third for other uses such as washing up, laundry, and food/drink preparation (see Table 3.1). It is notable that only a very small percentage of this potable standard water is actually drunk (1–2 L/hd.d). 3.2.2 Water–wastewater relationship As mentioned earlier, there is a strong link between water usage and wastewater disposal, with relatively little supplied water being “consumed” or taken out of the system. On a daily basis we can simply say Households (%) 16 14 12 10 8 6 4 2 0 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 Consumption (l/hd.d) Figure 3.2 Variation of per capita water consumption. (Based on Russac, D.A.V., et al. 1991. Journal of the Institution of Water and Environmental Management, 5(3), June, 342–351. With permission of the Chartered Institution of Water and Environmental Management, London.) 46 Urban Drainage Table 3.1 Percentage of water consumed for various purposes Water consumed (%) Component Household Commercial Industrial and agricultural WC flushing 27 35 5 Showering/bathing 28 26 1 Urinal flushing — 15 2 Food preparation/drinking 15 9 13 Laundry (washing machine) 16 8 — Washing-up (including 10 2 — dishwasher) Car washing/garden use 5 4 17 Other 1 1 62 Source: Adapted from Friedler, E. et al. 2013. Wastewater composition. Chapter 17 in Source Separation and Decentralization for Wastewater Management (eds.T.A. Larsen, K.M. Udert, and J. Lienert), IWA Publishing. G′ = xG (3.1) where G is the water consumption per person (L/hd.d), G′ is the wastewater generated per person (L/hd.d), and x is the return factor, given in Table 3.2 (–). It is estimated that, in the UK, about 95% of water used is returned to the sewer network. The other 5% is made up of water used externally (watering the garden and washing the car, for example) and to miscellaneous losses within the household. In hotter climates with low rainfall, this proportion can be up to 40%. Figure 3.3 shows a comparison made throughout the day between water use and waste- water flow in a catchment. In general, water use exceeds wastewater flow, especially in the early evening when gardens are being watered. At night this situation is reversed due to sewer infiltration flows. 3.2.3 Temporal variability It is emphasised that both wastewater quantity and quality vary widely from the very long term to the short term. Hence, any particular reported value should be related to the times- cale over which it was measured. Table 3.2 Percentage of water discharged as wastewater Country x (%) United Kingdom 95 Middle East Poor housing 85 Good housing 75 United States 60  Wastewater 47 Flow (l/hd.d) 400 Water Wastewater 300 200 100 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Time (h ) Figure 3.3 Typical diurnal plot of water consumption and wastewater flow. 3.2.3.1 Long term Until recently the long-term trend has been a steady increase in per capita consumption on an annual basis, reflecting a number of factors such as increased ownership of water-using domestic appliances. However, that trend has now been reversed downwards in the UK due to an increased emphasis on household water efficiency. 3.2.3.2 Annual Variations within the year due to seasonal effects can be observed in water demand. Evidence (Thackray et al., 1978) suggests that WC flushing decreases in summer (probably due to increased rate of body evaporation) and that bathing/showering increases. Outside water use increases significantly from gardening, and this can dominate the demand during summer months. For example, during the dry summer of 1995, increases in average demand of 50% or higher were observed in some areas. Figure 3.4 shows the monthly trends in the Anglian region for 3 years where the average consumption in July was up to 25% greater than in one of the winter months. The effect on wastewater flows is less clearly defined, but typically, summer dry weather flow discharges normally exceed winter flows by 10%–20%. 3.2.3.3 Weekly Variations in water demand and wastewater production can occur within the week, from day to day. Butler (1991) and Parker and Wilby (2013) found increased water consumption at weekends, probably due to increased WC flushing and bathing. This may also be due to a transfer of location rather than an increase per se. 3.2.3.4 Diurnal A basic diurnal pattern showing variation from hour to hour of wastewater is given in Figure 3.3. Minimum flows occur during the early morning hours when activity is at its 48 Urban Drainage Water consumption (l/hd.d) 170 1993 1994 1995 160 150 140 130 120 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Figure 3.4 Comparison of the effects on water consumption in a dry summer (Anglian region). lowest. The first peak generally occurs during the morning, the exact timing of which is dependent on the social activities of the community, but in this example, it is between 09:00 and 10:00. A second flow peak occurs in the early evening between 18:00 and 19:00, and then a third can also be distinguished between 21:00 and 22:00, but this is less clearly defined in magnitude and timing. Detailed timing within the diurnal cycle is also affected by the day of the week, with some differences noted at weekends. 3.2.4 Appliances Wastewater production is strongly linked to the widespread ownership and use of a wide range of domestic appliances, such as those in Table 3.3. The contribution of each individual appliance depends on both the volume of flow discharged after each operation and the fre- quency with which it is used. Table 3.3 shows typical discharge volumes of six different domestic appliances. Particularly large volumes are discharged by washing machines and during bathing, while relatively little is used during each use of the wash basin. Figure 3.5 illustrates how the discharges from the individual appliances go to make up the general wastewater diurnal pattern. The most important contributor overall is the WC, which although only of modest volume is used very frequently throughout the day, and par- ticularly at peak periods. Further discussion of the implications of the diurnal wastewater pattern is given in Chapter 8. 3.3 NON-DOMESTIC 3.3.1 Commercial This category includes businesses such as shops, offices, and light industrial units, and com- mercial establishments such as restaurants, laundries, public houses, and hotels.  Wastewater 49 Table 3.3 Average extant domestic appliance discharge volumes Appliance Volume (l)* WC (per flush) 9.5 Bath (per use) 80 Shower (per use) 35 Washing machine (per cycle) 80 Dishwasher (per cycle) 35 Source: After Lallana, C. et al. 2001. Sustainable Water Use in Europe, Part 2: Demand Management, Environmental Issue Report 19, European Environment Agency. * Introduction of low flow variants of these appliances will gradually reduce average volumes over time. Flow (l/hd.d) 400 Washing machine Shower 350 Bath 300 Wash basin Kitchen sink 250 WC 200 150 100 50 0 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00 Time (h ) Figure 3.5 Appliance diurnal discharge patterns. Demand is generated by drinking, washing, and sanitary facilities, but patterns of use are inevitably different to those generated by domestic usage. For example, Table 3.1 shows how toilet/urinal usage is an even more dominant component of water use (50%) than in the domestic environment. Much less detailed information is available on commercial usage than on domestic usage. 3.3.2 Industrial The component of wastewater generated by industrial processes can be important in specific situations but is more difficult to characterise in general because of the large variety of industries. Table 3.1 shows that many of the most important components of usage found in domestic and commercial premises are much less important in industry and agriculture. 50 Urban Drainage In most cases, effluents result from the following water uses: Sanitary (e.g., washing, drinking, personal hygiene) Processing (e.g., manufacture, waste and by-product removal, transportation) Cleaning Cooling The detailed rate of discharge will vary from industry to industry and will depend significantly on the actual processes used. Water consumption is often expressed in terms of volume used per mass of product. So, for example, papermaking consumes 50–150 m3/t and dairy products 3–35 m3/t. Industrial effluents can be highly variable (in both quantity and quality) as a consequence of batch discharges, operation start-ups and shut-downs, working hours, and other factors. These may change significantly at weekends. Depending on the relative magnitude of the flows, industrial discharges can completely alter the normal diurnal patterns of flow. There may also be significant seasonal changes in demand, for example, due to agro-industrial practices responding to the needs of food production. Other important factors include the size and structure of the organisation, the extent of process water recycling, and the availability and cost of water (Shang et al., 2016). 3.4 INFILTRATION AND INFLOW Unlike the other sources of wastewater, infiltration and inflow are not deliberate discharges but occur as a consequence of the existence of a piped network. Infiltration and inflow have been briefly introduced in Chapter 1 and are defined as water that enters the sewer system, including laterals and private sewers, through indirect and direct means, respectively. Infiltration is flow that enters the sewer system through defective drains and sewers (cracks and fissures), pipe joints, couplings, and manholes arising from Extraneous groundwater Spring water Seawater Water from other leaking pipes Inflow is actually stormwater that enters separate foul sewers from illegal or miscon- nected yard gullies, through roof downpipes, or through manhole covers (see Chapter 5 for further discussion on misconnections). This is extremely common, and a survey of one sepa- rate system (Inman, 1975) found that 40% of all houses had some arrangement whereby stormwater could enter the wastewater sewer. Inflow can also arise from adjacent storm sewers, land drainage, streams, and other watercourses. 3.4.1 Problems The presence of excessive amounts of infiltration may cause one or more of the following problems: Reduced effective sewer capacity leading to possible surcharging and/or flooding preventing the flushing of toilets, and so on, or pollution for long periods of time Overloading of pumping stations and wastewater treatment works  Wastewater 51 Higher frequency of combined sewer overflow operation, possibly in dry weather during periods of high groundwater levels Increased entry of sediment (soil), resulting in higher maintenance requirements and possible surface subsidence 3.4.2 Quantification The extent of infiltration is site specific, but when excessive, is usually a result of poor design and construction and will generally deteriorate as the system physically degrades. Influencing factors include the following (Bishop et al., 1998; Karpf and Krebs, 2013): Age of the system Standard of materials and methods Standard of workmanship in laying pipes Settlement due to ground movement Height of groundwater level (varies seasonally) Type of soil (hydraulic conductivity) Properties of the sewer trench Aggressive chemicals in the ground Extent of the network – total length of sewer (including house connections); type of pipe joint, number of joints and pipe size; number and size of manholes and inspection chambers Proximity of other drainage networks to transfer flow (e.g., laid in the same trench) Frequency of surcharge The amount of infiltration may range widely and can reach serious proportions in old systems. In the UK, rates up to 50% of average dry weather flow have been measured. The proportion of total infiltration arising from house connections may be up to 50% in places, but this is difficult to predict with any accuracy (Kohout et al., 2010). More complete details of the causes, costs, and control of infiltration can be found in White et al. (1997). 3.4.3 Exfiltration Exfiltration is the opposite of infiltration. Under certain circumstances, wastewater (or stormwater) is able to leak out of the sewer into the surrounding soil and groundwater. This creates the potential for infiltration into another system and groundwater contamina- tion, which could be critical in areas where an aquifer is used for drinking water supply. Rutsch et al. (2008) argue that there are contradictory viewpoints on whether or not exfiltration is a serious problem. Values of exfiltration (as with infiltration) are variable, with published rates as low as 1% and as high as 13% of dry weather flow in UK case studies (Rueedi et al., 2009; Wakida and Lerner, 2005), with even higher rates noted in unpublished studies. A study in Canada (Guérineau et al., 2014) found exfiltration to vary between 0.6 and 15.7 m3/d per kilometre during dry weather, and 1.1 and 19.5 m3/d per kilometre during wet weather. This variability is a result of the inherent differences across systems and of the many alternative methods of quantification used, many of which are indirect. Factors affecting the likelihood of exfiltration are similar to those discussed for infiltra- tion. More complete details of the causes, costs, control, and implications of exfiltration can be found in Anderson et al. (1996) and Reynolds and Barrett (2003). 52 Urban Drainage 3.5 WASTEWATER QUALITY Wastewater contains a complex mixture of natural organic and inorganic material present in various forms, from coarse grits, through fine suspended solids, to colloidal and soluble matter. Much is in the form of highly putrescible compounds. In addition, a small proportion of man-made substances, derived from commercial and industrial practices, will be present. In fact, wastewater is 99.9% water although the remaining 0.1% is significant, particu- larly if it is allowed to enter the environment. Fresh domestic wastewater is typically cloudy grey in colour with some recognisable solids and has a musty/soapy odour. With time (2–6 hours depending on ambient conditions), the waste “ages” and gradually changes in char- acter as a result of physical and biochemical processes. Stale wastewater is dark grey/black with smaller and fewer recognisable solids, and “older” flows can have a pungent “rotten eggs” odour due to the presence of hydrogen sulphide. Wastewater quality is variable in respect to both location and time. In addition, the techniques commonly used for sampling and analysis are subject to error (see Chapter 2). Therefore, caution is needed in interpreting standard or typical values. Such data should never be assumed to accurately represent the wastewater from a particular community – this can only be properly confirmed by a (possibly extensive) testing programme or access to historic data. 3.5.1 Pollutant sources Wastewater quality is influenced by the contaminants discharged into it derived mainly from human, household, and industrial activities. The quality of the carriage water (the original drinking water) or infiltrating groundwater can also be influential. 3.5.1.1 Human excreta Human excreta are responsible for a large proportion of the pollutants in wastewater. Adults produce on average 200–300 g of faeces and 1–1.5 kg of urine per day. Faeces account for 10–15 g/hd.d of biochemical oxygen demand (BOD) and urine 6 g/hd.d of BOD, but together only contribute a small proportion of wastewater fats (Friedler et al., 2013). Excreta are also an important source of nutrients. The bulk (94%) of the organic nitro- gen in wastewater is derived from excreta. Of this percentage, 50% derives from urine (urea), which is most abundant in fresh wastewater as it is rapidly converted to ammonia under both aerobic and anaerobic conditions (see Chapter 2). Approximately 50% of the phosphorus discharged to sewer (1.5 g/hd.d) is derived from excreta. Excreta also contain about 1 g/hd.d of sulphur (Gilmour et al., 2008). The bulk of the micro-organisms in wastewater originate in faeces; urine is relatively microbe free. 3.5.1.2 Toilet/WC A wide range of large (gross) solids is discharged, either deliberately or accidentally, via the toilet such as condoms, sanitary towels, panty liners, tampons, disposable diapers, toilet paper, paper towels, wet wipes, and cotton buds. Toilet paper is used in large quantities. Although this typically disintegrates in a matter of hours in the turbulent flow in sewers (Eren and Karadagli, 2012), it is only slowly biodegradable due to the presence of cellulose fibres. Spence et al. (2016) found by in-sewer sampling that 12–23 g/hd.d is disposed of influenced particularly by the demographics of the sampled catchments. Tests have shown  Wastewater 53 that coloured papers contribute some 15% of the wastewater chemical oxygen demand (COD). In total, some 0.15 sanitary items/hd are disposed of each day (Friedler et al., 1996). A number of cleaning, disinfecting, and descaling chemicals are also routinely discharged into the system via the toilet. 3.5.1.3 Food Digested food is the source of many of the excreta-related pollutants mentioned above. However, undigested food is a major contributor of fats, oils, and grease including butter, margarine, cooking oils, vegetable fats, meats, cereals, and nuts. Food residues are also a source of some organic nitrogen and phosphorus and of salt (NaCl). Food waste disposers are not widely used in the UK but are common in Australia, New Zealand, and the United States (Evans, 2012). Clearly, food waste load entering the sewer will be greater if this appliance is attached to the kitchen sink. Significant benefits (e.g., increased biogas production) are claimed on the basis of trials in Sweden, with very few drawbacks (e.g., increased water consumption, sewer deposition) in evidence (Evans et al., 2010; Mattsson et al., 2014). 3.5.1.4 Washing/laundry Washing and laundry activities add soaps and detergents to the sewer (e.g., washing machine and dishwater detergents). The polyphosphate builders used in synthetic detergents contrib- ute approximately 50% of the phosphorus load. Phosphorus concentrations have dimin- ished significantly in countries where legislation has imposed significant reductions in the amounts of phosphorus used by manufacturers of detergents (Morse et al., 1993). Values for total phosphorus are approximately 0.3 g/hd.d from laundry activity and 0.2 g/hd.d from dishwashers (Gilmour et al., 2008). 3.5.1.5 Industry The characteristics of industrial wastewaters, or trade effluents as they are often called, are similar to those of domestic wastewater in that they are likely to contain a very high propor- tion of water, and the impurities may be present as suspended, colloidal, or dissolved mat- ter. But in addition, a very large variety of pollutant types can be generated and industrial wastewater may contain Extremes of organic content A deficiency of nutrients Inhibiting chemicals (acids, toxins, bactericides) Resistant organic compounds Heavy metals and accumulative persistent organics Processing liquors from the main industrial processes tend to be relatively strong, while wastewaters from rinsing, washing, and condensing are comparatively weak. Discharges may be seasonal and vary considerably from day to day both in volume and strength. 3.5.1.6 Carriage water and groundwater The sulphate present in wastewater is derived principally from the mineral content of the municipal water supply or from saline groundwater infiltration (see Chapter 2). 54 Urban Drainage In hard water areas, the use of softeners can result in significant increases in the wastewater chloride concentrations. Infiltration of saltwater (if present) can contribute similarly. 3.5.2 Pollutant levels Typical values and ranges of pollutant levels in UK wastewater are given in Table 3.4. Table 3.4 Pollutant concentrations and unit loads for wastewater Parameter type Parameter Unit load (g/hd.d) Concentration (mg/L) mean (range) Physical Suspended solids Volatile 48 240 Fixed 12 60 Total 60 300 (180–450) Gross (sanitary) solids Sanitary refuse 0.15* Toilet paper 7 Temperature 18 (15–20) °C: summer 10°C: winter Chemical BOD5 Soluble 20 100 Particulate 40 200 Total 60 300 (200–400) COD Soluble 35 175 Particulate 75 375 Total 110 550 (350–750) TOC 40 200 (100–300) Nitrogen Organic N 4 20 Ammonia 8 40 Nitrites 0 Nitrates