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Costs of disposal of sewage sludge: a case study
133
Engineering Costs and Production Economics, 2 1 ( 199 1) I 33- I4 1 Elsevier
Costs of disposal of sewage sludge: A case study KC. Hau and D. Sculli University of Hong Kong, Hong Kong (Received January 16, 1989; accepted in revised form October 10, 1990)
Abstract This paper examines the ways and the costs of disposing sewage sludge and other associated solids. The objective of the overall plan is to minimize costs while maintaining an acceptable risk to human health, to living resources, and to public amenities. The current disposal methods are considered with reference to local constraints and environmental factors.
Introduction Most secondary sewage treatment processes separate domestic sewage into four components: effluent, coarse solids (screenings), grit, and sludge. The effluent is discharged into rivers or the sea at some acceptable rate depending on its quality. Screenings and grit represent only a small proportion of the total and are normally disposed of via sanitary landfills. Sewage sludge can be disposed through utilization in agriculture, by tipping in controlled landfill sites, by disposal at sea, or by incineration. The domestic sludge may be primary sludge and/or surplus digested or non-digested activated sludge. All sewage sludge from the major sewage treatment works in Hong Kong will be anaerobically digested, with solid contents ranging from 3 to 6%. The solid content can be increased by centrifuging to between 12% to 18%, by filter belt pressing to between 25% to 30%, or up to 3 5% by filter plate pressing. Because of Hong Kong’s small total area, livestock farming is highly concentrated, and livestock sludge disposal is also a problem (mainly from pigs and chickens). The quantity and solid content of agricultural sludge produced will vary over a wide range, depending on whether large farms and individual farmers will opt for “drymuck-out” or “wet-muck-out” keeping regions. Industrial sludge from factories and plants will
0167-188X/91 /$03.50 0 1991-Elsevier
Science Publishers B.V.
be produced in the designated industrial zones. This is treated in the same way as domestic sludge: dewatered by centrifuging or by filter belt pressing or by filter plate pressing. Hong Kong’s sewage sludge is currently being disposed of by tipping on controlled landfill sites. The daily quantities available from the sewage treatment works after dewatering by centrifuge to a 15% solid content averaged 100 m3/day in late 1984. A large increase is expected over the next 5 years; it is estimated that around 2,500 m3/ day of sewage sludge with 4% solid content will need to be disposed in 1993. Furthermore, agricultural waste sludge with a 4% solid content is expected to increase to around 1,500 m3/day by 1993, while alum sludge with 4% solid content in 1993 is estimated at 1,250 m3/day, see Yung and Jordan [ 11. All this will add new dimensions to the problem of sludge disposal. Sludge disposal methods employed by the European countries are: agricultural land, 40%; at sea, 26%; landfills, 25%; and by incineration. In Japan, where legislative control over disposal at sea are very stringent, the dominant method of disposal is by incineration. In the UK, 41% of sludge produced goes to agricultural land, where the proportion of agricultural land receiving sewage sludge is only about 1.3% of the total, see National Water Council [ 2 1. The potential for the disposal of sewage sludge to agricultural land (market garden type agriculture ) is severely lim-
Fig, I.Mapoftl[ongKong(3cm=8km).
ited in Hong Kong. It is limited by the insignificant amount of agricultural land available and by the suitability of sewage sludge itself for agricultural purposes. The disposal of large quantities of sewage sludge to a~cultural land is, therefore, not considered further. This leaves only 3 options of sewage sludge disposal in Hong Kong to be evaluated: Iandfill, sea disposal and incineration. The three largest sewage treatment plants expected to be built along Hong Kong’s coast-line are used for making a comparison of the 3 options on costs/benefit principles. These plants are denoted as Pl (Sha Tin), P2 (Tai PO) and P3 (North West Kowloon), see Fig. 1 for map. Disposal via landfills This method involves the spreading of sewage sludge over municipai solid general refuse dumps,
utilising the capacity of the general refuse to absorb water from the sewage sludge, see DiPinto and Mininni [ 3 ] . The sewage sludge will in turn be covered by another layer of general refuse, etc. It has been proved to be impractical to dispose liquid sludge with a moisture content greater than 80% in this manner. The method is practical only when the sludge is dewatered to about a 65% moisture content. Small scale trials on the disposal of sludge dewatered to 80% moisture content were carried out, and results showed that a mixing ratio of 5 tonnes of domestic waste to 1 tonne of sewage sludge is required for effective disposal via landfill methods. Any reduction in ratio appeared to give rise to difficulties in tipping operations. Hong Kong has only a few locations where this type of controlled tipping can be carried out. Disposal via landfill, unlike disposal to sea, is restricted by the lack of suitable tipping space.
135 Assuming the acceptable refuse/sludge mix ratio to be 1 to 5, the estimated sludge tipping capacity required is about 6 10 tonnes/day at a 20% solids contents - a very large figure for Hong Kong. The absorption of sludge depends on the depth of the tip refuse, stability of tip surface, total area, and tip drainage characteristics. When using this method, approximately one fifteenth of the tip volume will be occupied by sludge, so reducing the tip volume available for general refuse by the same amount. This figure is not large and thus will not seriously effect the general refuse tipping programme. However, if on site trial excavation tests are not favourable, for instance due to the type of refuse (certain types of industrial refuse may be less absorbent ), or to prolonged wet weather which may reverse the dewatering process, the assumption concerning the ratio will not be valid. Experience indicates that it is unlikely that the disposal programme will be immune from such effects. Care is required to avoid pollution of aquifiers or streams used for water abstraction or sport. There may also be odour emission, particularly during light wind periods, as well as the nuisance of flies. The routine placing of cover material by workers will be unpleasant and more difficult. Pathogenic bacteria may live for 5 weeks in dry weather and viruses up to 6 minutes in warm weather. However, the risk of the spread of infectious diseases will be very small when cautious practices are employed. Hong Kong has two sewage treatment plants that are away from the sea front, and that have TABLE 1 Estimates of quantity of sludge and costs for disposal to tips at 20% ($7.8 Hong Kong= l$ US)
Cost ($ millions) Capital Annual Replacement Quantity of sludge (tonnes/day at 20% d.s. or 80% of moisture content)
Sha Tin
Sha Tin and Tai PO
8.6 11.4 3.54
11.7 16.0 4.9
191
269
Sha Tin, Tai PO and N.W. Kowloon 20.8 30.6 8.5
513
filter plate presses installed. The sewage sludge from these plants, therefore, is a good candidate for disposal via landfill tippings. Sewage sludge from Hong Kong’s outlying islands and septic tanks is small and also suitable for landfill tipping on the islands themselves. Agricultural sludge and alum sludge, depending on future trials on sites and their locations, may also be able to be dewatered and disposed to landfill. The equipment employed in the landfill disposal of sludge will include dewatering equipment and skip transport lorries. The costs involved will then include the capital cost of the equipment, and the annual operating costs such as power, chemicals, labour, maintenance, fuel, covering material, and other site works. It is assumed that the lorries and skips will need to be replaced after 5 years of service, see National Water Council [ 4 1. The final cost figures are shown in Table 1. Disposal at sea Disposal at sea is a process of the once-andfor-all type. Figures for the UK show sea dumping to be popular, with percentage of dry solid varying from 2.5 to 6.5 and with a mean of 3.8, see National Water Council [ 2 1. It thus appears that the effectiveness of sewage sludge disposal to sea is not too sensitive to the concentration of solids. Experience in other countries also indicates that the nature and extent of the effects on the physical and chemical characteristics of the sea environment are difficult to ascertain, and the sphere of operation of such effects cannot be precisely and accurately determined. Several studies have been reported in this area, see National Water Council [ 5-71, Eagle et al. [ 8,9], Nor-ten etal. [lO,ll],andShelton [12]. High concentration of phosphate and ammonium-nitrogen will be toxic to marine organisms. The levels of phosphate and ammoniumnitrogen had been measured around Hong Kong waters. Unpolluted waters around Hong Kong had levels of both phosphate and ammonium-nitrogen below 20 mg/m3 (Yim and Fung, [ 131). In comparison with the concentration of ammonium-nitrogen of 50 mg/m3 which is likely to be toxic to many marine organisms, the present lev-
136 els are satisfactory. Upon sewage sludge disposal to the recommended dumping site, there can be a strong local effect producing high levels of such concentrations in the water. However, the fairly strong mid-water tidal current reaching 0.56 m/ s and 0.69 m/s in Summer and Winter Monsoon Periods respectively, together with eddy currents generated by the dumping vessel will be enable sufticient dilution by rapid mixing and dispersion in the mixed layer above the thermocline, see National Water Council [ 5 1. Assuming that the centre of mass of sludge settles to the sea bottom (about 30 m below sea level) at a speed of about 0.1 mm/s to 3 mm/s, the time required to dilute the sludge by a factor of 10 will be about 2 hrs to 24 hrs, see Callaway and Teeter [ 141. Therefore it may be concluded that the conditions of high levels of phosphate and ammonium-nitrogen are transient, being quickly reduced by dilution and dispersion. During the period from October to March, the disappearance of the thermocline and halocline in the Continental Shelf Area South of Hong Kong will enable vertical advection as well as horizontal advection to take place. Hence it is likely, prima facie, that more rapid dilution and dispersion of sludge disposed will happen during that period, see Norton et al. [ 111, Callaway and Teeter [ 141, and National Water Council [ 5 ] a It is therefore likely, from experience of other countries mentioned above and by comparing site conditions, that the mean concentrations of phosphate and ammonium-nitrogen are not beyond those found in Victoria Harbour and Aberdeen harbour (about 40 mg/m3), see National Water Council [ 5 1, Eagle et al. [ 9 1, and Norton etal. ill]. Low concentrations of dissolved oxygen will also be harmful to marine organisms. The concentration of dissolved oxygen 3 m above the bottom of the recommended dumping site had a mean which sometimes reached slightly over 50%. Since a healthy environment for marine organisms requires an oxygen concentration of 50% or more, the present situation of dissolved oxygen content appears satisfactory, see National Water Council [ 5 1. Upon sewage sludge disposal to the recommended dumping site, deoxygenation of the receiving water caused by the high
BOD of sewage sludge, though not readily detectable in most of the dumping sites in other countries mentioned above, may be possible during several months. It is only possible to specuiate how serious the problem may be by extensive knowledge about the bottom sediments, bed mobility, bottom water drift, and the settling characteristics of sludge disposed during that period. The accumulation of organic matter at the bottom of the sea will aggravate the deoxygenation effect. The bottom sediments at the recommended dumping site are likely to be mud and sand which will tend to retain organic matters, see Shin and Thompson [ 15 1. The bottom water drift had a mean of about 0.09 m/s and sometimes reached 0.22 m/s during the observed period of possible deoxygenation, which is more than double the mean bottom water drift in Liverpool Bay of 0.04 m/s, see National Water Council [ 5 1. The comparison of the recommended dumping site condition with that in Liverpool Bay suggests that the hydrographic conditions inducing bed mobility will carry away the permanent accumulation of organic mud close to the sludge disposal site and hence alleviate the deoxygenation problem there. The likely accumulation of dumping material in the area of the density gradient surface (a narrow zone at the bottom of the thermocline) before settling out gradually will alleviate the situation of rapid organic matter build-up at the bottom of the site, hence it will further alleviate the deoxygenation problem there. Heavy metals, such as copper, zinc, lead, chromium and mercury may settle in particular organic phase in sediments at the centre of the dumping area, and are thus locked up in the muddier areas. The existing concentrations of copper, lead, zinc and chromium were found to be below 28,26,79 and 20 ppm, respectively, see Yim and Fung [ 13 1. The heavy metal content of the sewage sludge, though likely to be low, will contribute to the cumulative concentration. However, owing to bed mobility and bottom water drift, it is speculated that these heavy metal sediments will also be carried away from the dumping area, see Stemberg [ 16 1, Watts [ 17.18 ] and Williamson [ 19 1. The estimation is from the experience of other
137 TABLE 2 Estimates of quantity of sludge and costs for disposal Kong waters at 4% d.s. ($7.8 Hong Kong=$l US)
Quantity of sludge (Tonnes/day at 4% d.s. ) Options
Cost ($ milhons) Capital 4nnual Replacement
to Hong
Sha Tin
Sha Tin and Tai PO
Sha Tin, Tai PO and N.W. Kowloon
937
1,345
2,565
1 no. 600 tonne vessel
I no. 600 tonne vessel
2 nos. 600 tonne vessels
56.52 6.01 56.52
56.52 8.01 56.52
113.04 15.52 113.04
countries, see National Water Council [ 5 1, Norton et al. [ 11 1, Eagle et al. [ 8,9], and Shelton [ 12 1, and by comparing site conditions, that the heavy metal sediments are likely to be limited to an area of about 10 miles with the dumping site as centre and that the maximum concentrations of metals will not exceed 8 times the background concentration. Research indicates that there may be a richer concentration of phytoplankton in areas of high concentration of organic carbon and dissolved oxygen. On the other hand, heavy metal, chlorinated hydrocarbon and low nitrogen to phosphate ratio are unlikely to promote phanktonic algal growths. There is uncertainty about the effects of sewage sludge on marine processes and the fate and
TABLE Estimates
pathways of contaminants including metals, as well as the role, if any, of sewage sludge in the formation of algal blooms and in fish disease. It is thus essential to constantly monitor the effect of sewage sludge dumping so as to ensure the protection of the marine environment when disposal to sea is the option chosen, see Shelton [ 201 and National Water Council [ 5 1. The disposal to sea method is preferable for the sewage sludge from treatment plants PI, P2 and P3. The existing dewatering equipment and those under construction will no longer be required except for emergency purposes or during maintenance periods for the sea dumping vessels. Separate arrangement of some of this equipment for the disposal to landfill and other purposes may be possible. Since 1975, marine dumping in Hong Kong waters has been subject to the provisions of the Dumping at Sea (Overseas) Territories Order. The capacity of the existing dumping sites appears to fall short of the dumping requirements of projects in the next 5 years. Therefore, evaluation of the recommended sites with respect to the effect of sewage sludge disposal has been made. This method involves the dumping of sewage sludge to a dumping ‘site’ east of Hong Kong’s sea waters and at least 15 km from the coastline, by means of vessels of capacities ranging from 600 to 1,200 tonnes. These vessels will leave for the dumping site from the 3 proposed sewage treatment plants. Marine feasibility studies were carried out over
3 of costs for disposal
to international
waters
Sha Tin Options
Cost ($ milltons) Capital r\nnual Replacement
at 4% ds.
($7.8 Hong
Kong=$
Sha Tin and Tai PO
2 nos. 600 t. vessels
1 no. 1,200 t. vessel
2 nos. 600 t. vessels
I no. 1,200 1. vessel
81.05 15.44 81.05
104.77 9.51 104.77
81.05 20.59 81.05
104.77 12.68 104.77
I US) Sha Tin, Tai PO and N.W. Kowloon 4 nos. 600 t. vessels
162.10 40.75 162.10
2 nos. 600 t. vessels & I no. 1,200 t. vessel
2 nos. 1,200 1. vessels
185.82 33.26 185.82
209.54 25.35 209.54
138 the summer and winter monsoon periods. Measurements recorded include, current flow, winds, salinity, temperature, dissolved oxygen. The amount of both turbidity and suspended solids were recorded at three depths at three surveying points at flood and ebb cycles for spring and neap tides. Equipment used included direct reading current meters, temperature/salinity/dissolved oxygen meters, water sampling equipment together with floats, turbidity meter, micro-wave position fixing equipment, and other laboratory equipment, see Yung and Jordan [ 11. Analysis of the data collected showed that the dumping site will be suitable, at least for short term purposes, for sludge disposal with a 4% solids content. It can also be assumed that dumping sites farther out, where general conditions are more suitable, can also be used at higher transportation costs. The costs involved will include the capital costs of the vessels, dredging and terminal works, labour, fuel and maintenance, and the replacement costs of vessels and other facilities after an assumed 20 years of service, see National Water Council [ 41. The estimated costs involved for disposal to sea from the 3 source points are shown in Tables 2 and 3. Incineration
emissions are likely to occur. Vapour plumes are also often mistaken for smoke by the general public. The incineration plant may also need to be large and “obstructive” and could be environmentally unacceptable. The incineration of sludge is generaliy considered as a complemental option, for example, some specially difficult sludges prevent the use of other disposal options and must be disposed of via incineration. This method involves the incineration of a mixture of general municipal waste and sewage sludge at 30% solids content, reducing the bulk of sludge to a small volume of relatively inert ash amounting to 10 to 20% of the weight of feed cake burned. Incinerator plants can be multiple hearth furnaces or fluidized bed incinerators. The costs involved will include capital costs of the incineration plant, annual costs covering labour, fuel, chemicals, power and maintenance, and replacement costs of electrical and mechanical equipment after 15 years of service, see National Water Council [ 41. A site on Hong Kong’s east coast has been identified as a probable candidate for an incinerator plant. After making the necessary assumptions concerning quantity and quality of sludge, the costs of disposal by incineration at the proposed plant from the 3 sources under consideration were computed, see Table 4. Overall economics and concluding comments
The incineration of sewage sludge also has problems of environmental acceptability. Environmental nuisances such as dust, grit and odour TABLE 4 Estimates of quantity of sludge and costs for disposal by incineration at 30°h d.s. ($7.8 Hong Kong=$l US)
Quantity of sludge (tonnes/day at 30% d.s. ) Option
Cost ($ millions) Capital Annual Replacement
Sha Tin
Sha Tin and Tai PO
127
179
Incineration plant at 200 t/day
incineratjon plant at 280 t/day
90 12.7 12
130 17.8 104
The analysis assumed that sludge is increasing at a steady rate, and the 1993 forecast figure is used as the ultimate quantity. Based on the assumptions mentioned, the relative costs of the various options are compared in terms of their cumulative net present values (NPV) using a 5% discount rate. The NPV’s of the options over a planning period of 20 years are summarized in Tables 5,6 and 7. The results support the following conclusions: ( 1) The cumulative NPV’s for disposal to Hong Kong waters are always lower than the respective cumulative NPV’s for disposal to international waters. This conclusion is rather obvious since the distance involved is much less, while other factors remain equal. It is also the least expensive means of sludge disposal among the various alternatives. (2) The cumulative NPV’s for incineration are
139 TABLE 5 Comparison
of options Sha Tin sewage treatment
Options
Disposal to Hong Kong waters
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
($7.8 Hong Kong=$l
US)
Disposal to international
waters
Disposal to tips
Incineration
2 no. 600 t vessels
1 no. 1,200 t vessel
108.60
217.90
186.50
118.40
234.40
1.00
2.01
1.72
1.09
2.16
TABLE 6 Comparison
of options Sha Tin and Tai PO sewage treatment
Options
Disposal to Hong Kong waters
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
($7.8 Hong Kong=$l
Disposal to international
US)
waters
Disposal to tips
Incineration
2 no. 600 t vessels
1 no. 1,200 t vessel
126.80
270.40
215.30
165.70
333.60
1.oo
2.13
1.70
1.31
2.63
TABLE 7 Comparison
of options Sha Tin, Tai PO and N.W. Kowloon ($7.8 Hong Kong=$l
Options
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
Disposal to Hong Kong waters
Disposal to international
US) Disposal to tips
waters
4 nos. 600 t vessel
2 nos. 600 t vessel and 1 no. 1,200 t vessel
2 nos. 1,200 t vessels
249.10
525.80
480.00
430.60
313.90
1.oo
2.11
1.93
1.73
1.26
always higher than the respective cumulative NPV’s for the other options. Hence, incineration is likely to be the most expensive option. (3) The cumulative NPV’s for disposal to landfill tips is the second best option, while the ratio of cumulative NPV of disposal to tips with respect to disposal to Hong Kong waters increases as the sludge quantity from same source increases. Hence disposal to tips is a limited option and should be used sparingly. A final decision on the method of disposal must also take into account environmental and ‘social
costs’, and not just the monetary costs. Many of the environmental effects, particularly those concerned with disposal at sea, are difficult to quantify and are not well understood. The social awareness of the general public must also be considered, since a method like incineration, which may not be environmentally damaging, will still ‘invite’ comments from the general public on its visual aspects. Dumping at sea will not be under the eye of the general public, and will be socially acceptable even if its environmental aspects may be more damaging.
140
The disposal of sludge to Hong Kong waters is sound from both operational and economic standpoints, at least in the short term. It is likely to be the least expensive among all options considered. Dumping in international waters is also acceptable at a slightly higher transportation cost. The disposal of sludge to international waters is also likely to be sound from a long term operational standpoint. It is likely to be the third least expensive among all options considered, but political advice and more site tests are required before this option can be chosen. The disposal of sludge to controlled tips is the second least expensive among all options considered. It is unlikely, if that option is chosen, that the solid waste disposal programme will be immune from the effects of reduced volume of tips for refuse. This is particularly suitable for sewage sludge from inland treatment plants. It is also suitable for agricultural sludge and alum sludge, and for the small amounts of sludge generated in the outlying islands. Incineration of sludge is likely to be the most expensive among all options considered. It should be adopted only where special circumstances justify its use, and hence only for small disposal quantities. It may also be desirable to maintain flexibility by having more than one type of major outlet, or even a combination of different outlets. The large majority of sewage sludge should, of course, be disposed at lowest cost in order to achieve overall efficiency. This should be done with due consideration of risk to human health, harm to living resources, damage to amenities, and restriction of normal activities of people. This case study has, in the main, been concerned with the establishment of general procedures to estimate the costs of the various options while maintaining an acceptable level of risk to the environment. The real life situation is, of course, dynamic, with costs constantly changing through technology improvements and the relaxation or tightening of other constraints. Sensitivity and other mathematical analysis using existing cost tigures may, therefore, turn out to be more of academic than of real interest. This study has only attempted to provide a general framework that can be used to quickly determine the economics involved at any point in time.
References
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5 6
7
8
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11
12 13
14
Yung, K.K. and Jordan, R.W., 1985. Development of an integrated strategy for the management of sludges in Hong Kong. IWPC Seminar, Hong Kong, December 1985. National Water Council, 1977. Report of the sub-committee on the disposal of sewage sludge to land. Department of the Environment, STC Report No. 20, London, UK. DiPinto, A.C. and Mininni, G., 1985. Optimization of processing, managing and disposing of sewage sludge, Part II: disposal options. In: Processing and Usage of Organic Sludge and Liquid Agricultural Wastes, L’hermite. P., (Ed.), D. Reidel Publishing Co., The Netherlands. National Water Council, 198 I. Report of the sub-committee on the economics of sewage sludge disposal. Department of the Environment, London, UK. National Water Council, 1972. Out of sight out of mind, Department of the Environment. HMSO, London, UK. National Water Council, 1979. Report of the sub-committee on the disposal of sewage sludge to sea 1975-78. Department of the Environment, STC Report No. 18, London, UK. National Water Council, 1981. Report of the standing committee of the disposal of sewage sludge. Department of the Environment, UK. Eagle, R.A., Norton, M.G., Nunny, R.S. and Rolfe, MS., 1978. The field assessment of effects of dumping wastes at sea - 2: methods. Fisheries Research Technical Report No. 47, Ministry of Agriculture, Fisheries and Food. Lowestoft. Eagle, R.A., Hardiman, P.A., Norton, M.G. and Nunny, R.S., 1978. The field assessment of the effects of dumping wastes at sea - 3: a survey of the sewage sludge disposal area in Lyme Bay. Fisheries Research Technical Report No. 49, Ministry of Agriculture, Fisheries and Food, Lowestoft. Norten, M.G. and Rolf, M.S., 1978. The field assessment effects of dumping wastes at sea - I: an introduction. Fisheries Research Technical Report No. 45, Ministry of Agriculture, Fisheries and Food, Lowestoft. Norton, M.G., Eagle, R.A., Nunny, R.S., Rolfe, M.S., Hardiman, P.A. and Hampton, B.L., 198 1. The field assessment of the effects of dumping wastes at sea: 8: sewage sludge dumping in the outer Thames Estuary. Fisheries Research Technical Report No. 62. Ministry of Agriculture, Fisheries and Food, Lowestoft. Shelton, R.G.J., 197 1. Sludge dumping in the thames estuary. Mar. Pollut. Bull., 2: 24-27. Yim, W.W.S. and Fung. K.W., 1981. Heavy metals in marine sediments of Hong Kong. Hong Kong Eng., October 1981. Callaway, R., Teeter, A.M., Browne, D.W. and Ditsworh, G.R., 1976. Preliminary analysis of the dispersion of sewage sludge discharged from vessels to New York Bight Waters, Middle Atlantic, Continental Shelf and the New
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York Bight, Gross M., (Ed. ), Spec. Symp. Am. Sot. Limnol. Oceanogr., 2: 199-2 11. Shin, P.K.S. and Thompson, G.B., 1982. Spatial distribution of infaunal benthos of Hong Kong. Mar. Ecol. Prog. Ser., 10: 37-47. Stemberg, R.W., 1972. Predicting initial motion and bedload transport of sediment particles in the shallow marine environment. Shelf Sediment Transport: Process and Pattern, Scrift, D.J.P. et al. (Eds.), Hutchison and Ross Inc., Strondsburg, USA. Watts, J.C.D., 1970. The hydrology of the continental shelf area south of Hong Kong. Oceanographic Obser-
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vations for the year 1969, Hong Kong Fisheries Bull., 1: 51-57. Watts, J.C.D., 1973. Further observations on the hydrology of the Hong Kong territorial waters. Hong Kong Fisheries Bull., 3: 9-35. Williamson, G.R., 1970. Hydrography and weather of the Hong Kong fishing grounds. Hong Kong Fisheries Bull., 1: 43-48. Shelton, R.G.J., 1973. Some effects of dumped solid wastes on marine life and fisheries. North Sea Science, Goldberg, E.D. (Ed.), MIT Press, Cambridge, MA and London, pp. 415-436.
Engineering Costs and Production Economics, 2 1 ( 199 1) I 33- I4 1 Elsevier
Costs of disposal of sewage sludge: A case study KC. Hau and D. Sculli University of Hong Kong, Hong Kong (Received January 16, 1989; accepted in revised form October 10, 1990)
Abstract This paper examines the ways and the costs of disposing sewage sludge and other associated solids. The objective of the overall plan is to minimize costs while maintaining an acceptable risk to human health, to living resources, and to public amenities. The current disposal methods are considered with reference to local constraints and environmental factors.
Introduction Most secondary sewage treatment processes separate domestic sewage into four components: effluent, coarse solids (screenings), grit, and sludge. The effluent is discharged into rivers or the sea at some acceptable rate depending on its quality. Screenings and grit represent only a small proportion of the total and are normally disposed of via sanitary landfills. Sewage sludge can be disposed through utilization in agriculture, by tipping in controlled landfill sites, by disposal at sea, or by incineration. The domestic sludge may be primary sludge and/or surplus digested or non-digested activated sludge. All sewage sludge from the major sewage treatment works in Hong Kong will be anaerobically digested, with solid contents ranging from 3 to 6%. The solid content can be increased by centrifuging to between 12% to 18%, by filter belt pressing to between 25% to 30%, or up to 3 5% by filter plate pressing. Because of Hong Kong’s small total area, livestock farming is highly concentrated, and livestock sludge disposal is also a problem (mainly from pigs and chickens). The quantity and solid content of agricultural sludge produced will vary over a wide range, depending on whether large farms and individual farmers will opt for “drymuck-out” or “wet-muck-out” keeping regions. Industrial sludge from factories and plants will
0167-188X/91 /$03.50 0 1991-Elsevier
Science Publishers B.V.
be produced in the designated industrial zones. This is treated in the same way as domestic sludge: dewatered by centrifuging or by filter belt pressing or by filter plate pressing. Hong Kong’s sewage sludge is currently being disposed of by tipping on controlled landfill sites. The daily quantities available from the sewage treatment works after dewatering by centrifuge to a 15% solid content averaged 100 m3/day in late 1984. A large increase is expected over the next 5 years; it is estimated that around 2,500 m3/ day of sewage sludge with 4% solid content will need to be disposed in 1993. Furthermore, agricultural waste sludge with a 4% solid content is expected to increase to around 1,500 m3/day by 1993, while alum sludge with 4% solid content in 1993 is estimated at 1,250 m3/day, see Yung and Jordan [ 11. All this will add new dimensions to the problem of sludge disposal. Sludge disposal methods employed by the European countries are: agricultural land, 40%; at sea, 26%; landfills, 25%; and by incineration. In Japan, where legislative control over disposal at sea are very stringent, the dominant method of disposal is by incineration. In the UK, 41% of sludge produced goes to agricultural land, where the proportion of agricultural land receiving sewage sludge is only about 1.3% of the total, see National Water Council [ 2 1. The potential for the disposal of sewage sludge to agricultural land (market garden type agriculture ) is severely lim-
Fig, I.Mapoftl[ongKong(3cm=8km).
ited in Hong Kong. It is limited by the insignificant amount of agricultural land available and by the suitability of sewage sludge itself for agricultural purposes. The disposal of large quantities of sewage sludge to a~cultural land is, therefore, not considered further. This leaves only 3 options of sewage sludge disposal in Hong Kong to be evaluated: Iandfill, sea disposal and incineration. The three largest sewage treatment plants expected to be built along Hong Kong’s coast-line are used for making a comparison of the 3 options on costs/benefit principles. These plants are denoted as Pl (Sha Tin), P2 (Tai PO) and P3 (North West Kowloon), see Fig. 1 for map. Disposal via landfills This method involves the spreading of sewage sludge over municipai solid general refuse dumps,
utilising the capacity of the general refuse to absorb water from the sewage sludge, see DiPinto and Mininni [ 3 ] . The sewage sludge will in turn be covered by another layer of general refuse, etc. It has been proved to be impractical to dispose liquid sludge with a moisture content greater than 80% in this manner. The method is practical only when the sludge is dewatered to about a 65% moisture content. Small scale trials on the disposal of sludge dewatered to 80% moisture content were carried out, and results showed that a mixing ratio of 5 tonnes of domestic waste to 1 tonne of sewage sludge is required for effective disposal via landfill methods. Any reduction in ratio appeared to give rise to difficulties in tipping operations. Hong Kong has only a few locations where this type of controlled tipping can be carried out. Disposal via landfill, unlike disposal to sea, is restricted by the lack of suitable tipping space.
135 Assuming the acceptable refuse/sludge mix ratio to be 1 to 5, the estimated sludge tipping capacity required is about 6 10 tonnes/day at a 20% solids contents - a very large figure for Hong Kong. The absorption of sludge depends on the depth of the tip refuse, stability of tip surface, total area, and tip drainage characteristics. When using this method, approximately one fifteenth of the tip volume will be occupied by sludge, so reducing the tip volume available for general refuse by the same amount. This figure is not large and thus will not seriously effect the general refuse tipping programme. However, if on site trial excavation tests are not favourable, for instance due to the type of refuse (certain types of industrial refuse may be less absorbent ), or to prolonged wet weather which may reverse the dewatering process, the assumption concerning the ratio will not be valid. Experience indicates that it is unlikely that the disposal programme will be immune from such effects. Care is required to avoid pollution of aquifiers or streams used for water abstraction or sport. There may also be odour emission, particularly during light wind periods, as well as the nuisance of flies. The routine placing of cover material by workers will be unpleasant and more difficult. Pathogenic bacteria may live for 5 weeks in dry weather and viruses up to 6 minutes in warm weather. However, the risk of the spread of infectious diseases will be very small when cautious practices are employed. Hong Kong has two sewage treatment plants that are away from the sea front, and that have TABLE 1 Estimates of quantity of sludge and costs for disposal to tips at 20% ($7.8 Hong Kong= l$ US)
Cost ($ millions) Capital Annual Replacement Quantity of sludge (tonnes/day at 20% d.s. or 80% of moisture content)
Sha Tin
Sha Tin and Tai PO
8.6 11.4 3.54
11.7 16.0 4.9
191
269
Sha Tin, Tai PO and N.W. Kowloon 20.8 30.6 8.5
513
filter plate presses installed. The sewage sludge from these plants, therefore, is a good candidate for disposal via landfill tippings. Sewage sludge from Hong Kong’s outlying islands and septic tanks is small and also suitable for landfill tipping on the islands themselves. Agricultural sludge and alum sludge, depending on future trials on sites and their locations, may also be able to be dewatered and disposed to landfill. The equipment employed in the landfill disposal of sludge will include dewatering equipment and skip transport lorries. The costs involved will then include the capital cost of the equipment, and the annual operating costs such as power, chemicals, labour, maintenance, fuel, covering material, and other site works. It is assumed that the lorries and skips will need to be replaced after 5 years of service, see National Water Council [ 4 1. The final cost figures are shown in Table 1. Disposal at sea Disposal at sea is a process of the once-andfor-all type. Figures for the UK show sea dumping to be popular, with percentage of dry solid varying from 2.5 to 6.5 and with a mean of 3.8, see National Water Council [ 2 1. It thus appears that the effectiveness of sewage sludge disposal to sea is not too sensitive to the concentration of solids. Experience in other countries also indicates that the nature and extent of the effects on the physical and chemical characteristics of the sea environment are difficult to ascertain, and the sphere of operation of such effects cannot be precisely and accurately determined. Several studies have been reported in this area, see National Water Council [ 5-71, Eagle et al. [ 8,9], Nor-ten etal. [lO,ll],andShelton [12]. High concentration of phosphate and ammonium-nitrogen will be toxic to marine organisms. The levels of phosphate and ammoniumnitrogen had been measured around Hong Kong waters. Unpolluted waters around Hong Kong had levels of both phosphate and ammonium-nitrogen below 20 mg/m3 (Yim and Fung, [ 131). In comparison with the concentration of ammonium-nitrogen of 50 mg/m3 which is likely to be toxic to many marine organisms, the present lev-
136 els are satisfactory. Upon sewage sludge disposal to the recommended dumping site, there can be a strong local effect producing high levels of such concentrations in the water. However, the fairly strong mid-water tidal current reaching 0.56 m/ s and 0.69 m/s in Summer and Winter Monsoon Periods respectively, together with eddy currents generated by the dumping vessel will be enable sufticient dilution by rapid mixing and dispersion in the mixed layer above the thermocline, see National Water Council [ 5 1. Assuming that the centre of mass of sludge settles to the sea bottom (about 30 m below sea level) at a speed of about 0.1 mm/s to 3 mm/s, the time required to dilute the sludge by a factor of 10 will be about 2 hrs to 24 hrs, see Callaway and Teeter [ 141. Therefore it may be concluded that the conditions of high levels of phosphate and ammonium-nitrogen are transient, being quickly reduced by dilution and dispersion. During the period from October to March, the disappearance of the thermocline and halocline in the Continental Shelf Area South of Hong Kong will enable vertical advection as well as horizontal advection to take place. Hence it is likely, prima facie, that more rapid dilution and dispersion of sludge disposed will happen during that period, see Norton et al. [ 111, Callaway and Teeter [ 141, and National Water Council [ 5 ] a It is therefore likely, from experience of other countries mentioned above and by comparing site conditions, that the mean concentrations of phosphate and ammonium-nitrogen are not beyond those found in Victoria Harbour and Aberdeen harbour (about 40 mg/m3), see National Water Council [ 5 1, Eagle et al. [ 9 1, and Norton etal. ill]. Low concentrations of dissolved oxygen will also be harmful to marine organisms. The concentration of dissolved oxygen 3 m above the bottom of the recommended dumping site had a mean which sometimes reached slightly over 50%. Since a healthy environment for marine organisms requires an oxygen concentration of 50% or more, the present situation of dissolved oxygen content appears satisfactory, see National Water Council [ 5 1. Upon sewage sludge disposal to the recommended dumping site, deoxygenation of the receiving water caused by the high
BOD of sewage sludge, though not readily detectable in most of the dumping sites in other countries mentioned above, may be possible during several months. It is only possible to specuiate how serious the problem may be by extensive knowledge about the bottom sediments, bed mobility, bottom water drift, and the settling characteristics of sludge disposed during that period. The accumulation of organic matter at the bottom of the sea will aggravate the deoxygenation effect. The bottom sediments at the recommended dumping site are likely to be mud and sand which will tend to retain organic matters, see Shin and Thompson [ 15 1. The bottom water drift had a mean of about 0.09 m/s and sometimes reached 0.22 m/s during the observed period of possible deoxygenation, which is more than double the mean bottom water drift in Liverpool Bay of 0.04 m/s, see National Water Council [ 5 1. The comparison of the recommended dumping site condition with that in Liverpool Bay suggests that the hydrographic conditions inducing bed mobility will carry away the permanent accumulation of organic mud close to the sludge disposal site and hence alleviate the deoxygenation problem there. The likely accumulation of dumping material in the area of the density gradient surface (a narrow zone at the bottom of the thermocline) before settling out gradually will alleviate the situation of rapid organic matter build-up at the bottom of the site, hence it will further alleviate the deoxygenation problem there. Heavy metals, such as copper, zinc, lead, chromium and mercury may settle in particular organic phase in sediments at the centre of the dumping area, and are thus locked up in the muddier areas. The existing concentrations of copper, lead, zinc and chromium were found to be below 28,26,79 and 20 ppm, respectively, see Yim and Fung [ 13 1. The heavy metal content of the sewage sludge, though likely to be low, will contribute to the cumulative concentration. However, owing to bed mobility and bottom water drift, it is speculated that these heavy metal sediments will also be carried away from the dumping area, see Stemberg [ 16 1, Watts [ 17.18 ] and Williamson [ 19 1. The estimation is from the experience of other
137 TABLE 2 Estimates of quantity of sludge and costs for disposal Kong waters at 4% d.s. ($7.8 Hong Kong=$l US)
Quantity of sludge (Tonnes/day at 4% d.s. ) Options
Cost ($ milhons) Capital 4nnual Replacement
to Hong
Sha Tin
Sha Tin and Tai PO
Sha Tin, Tai PO and N.W. Kowloon
937
1,345
2,565
1 no. 600 tonne vessel
I no. 600 tonne vessel
2 nos. 600 tonne vessels
56.52 6.01 56.52
56.52 8.01 56.52
113.04 15.52 113.04
countries, see National Water Council [ 5 1, Norton et al. [ 11 1, Eagle et al. [ 8,9], and Shelton [ 12 1, and by comparing site conditions, that the heavy metal sediments are likely to be limited to an area of about 10 miles with the dumping site as centre and that the maximum concentrations of metals will not exceed 8 times the background concentration. Research indicates that there may be a richer concentration of phytoplankton in areas of high concentration of organic carbon and dissolved oxygen. On the other hand, heavy metal, chlorinated hydrocarbon and low nitrogen to phosphate ratio are unlikely to promote phanktonic algal growths. There is uncertainty about the effects of sewage sludge on marine processes and the fate and
TABLE Estimates
pathways of contaminants including metals, as well as the role, if any, of sewage sludge in the formation of algal blooms and in fish disease. It is thus essential to constantly monitor the effect of sewage sludge dumping so as to ensure the protection of the marine environment when disposal to sea is the option chosen, see Shelton [ 201 and National Water Council [ 5 1. The disposal to sea method is preferable for the sewage sludge from treatment plants PI, P2 and P3. The existing dewatering equipment and those under construction will no longer be required except for emergency purposes or during maintenance periods for the sea dumping vessels. Separate arrangement of some of this equipment for the disposal to landfill and other purposes may be possible. Since 1975, marine dumping in Hong Kong waters has been subject to the provisions of the Dumping at Sea (Overseas) Territories Order. The capacity of the existing dumping sites appears to fall short of the dumping requirements of projects in the next 5 years. Therefore, evaluation of the recommended sites with respect to the effect of sewage sludge disposal has been made. This method involves the dumping of sewage sludge to a dumping ‘site’ east of Hong Kong’s sea waters and at least 15 km from the coastline, by means of vessels of capacities ranging from 600 to 1,200 tonnes. These vessels will leave for the dumping site from the 3 proposed sewage treatment plants. Marine feasibility studies were carried out over
3 of costs for disposal
to international
waters
Sha Tin Options
Cost ($ milltons) Capital r\nnual Replacement
at 4% ds.
($7.8 Hong
Kong=$
Sha Tin and Tai PO
2 nos. 600 t. vessels
1 no. 1,200 t. vessel
2 nos. 600 t. vessels
I no. 1,200 1. vessel
81.05 15.44 81.05
104.77 9.51 104.77
81.05 20.59 81.05
104.77 12.68 104.77
I US) Sha Tin, Tai PO and N.W. Kowloon 4 nos. 600 t. vessels
162.10 40.75 162.10
2 nos. 600 t. vessels & I no. 1,200 t. vessel
2 nos. 1,200 1. vessels
185.82 33.26 185.82
209.54 25.35 209.54
138 the summer and winter monsoon periods. Measurements recorded include, current flow, winds, salinity, temperature, dissolved oxygen. The amount of both turbidity and suspended solids were recorded at three depths at three surveying points at flood and ebb cycles for spring and neap tides. Equipment used included direct reading current meters, temperature/salinity/dissolved oxygen meters, water sampling equipment together with floats, turbidity meter, micro-wave position fixing equipment, and other laboratory equipment, see Yung and Jordan [ 11. Analysis of the data collected showed that the dumping site will be suitable, at least for short term purposes, for sludge disposal with a 4% solids content. It can also be assumed that dumping sites farther out, where general conditions are more suitable, can also be used at higher transportation costs. The costs involved will include the capital costs of the vessels, dredging and terminal works, labour, fuel and maintenance, and the replacement costs of vessels and other facilities after an assumed 20 years of service, see National Water Council [ 41. The estimated costs involved for disposal to sea from the 3 source points are shown in Tables 2 and 3. Incineration
emissions are likely to occur. Vapour plumes are also often mistaken for smoke by the general public. The incineration plant may also need to be large and “obstructive” and could be environmentally unacceptable. The incineration of sludge is generaliy considered as a complemental option, for example, some specially difficult sludges prevent the use of other disposal options and must be disposed of via incineration. This method involves the incineration of a mixture of general municipal waste and sewage sludge at 30% solids content, reducing the bulk of sludge to a small volume of relatively inert ash amounting to 10 to 20% of the weight of feed cake burned. Incinerator plants can be multiple hearth furnaces or fluidized bed incinerators. The costs involved will include capital costs of the incineration plant, annual costs covering labour, fuel, chemicals, power and maintenance, and replacement costs of electrical and mechanical equipment after 15 years of service, see National Water Council [ 41. A site on Hong Kong’s east coast has been identified as a probable candidate for an incinerator plant. After making the necessary assumptions concerning quantity and quality of sludge, the costs of disposal by incineration at the proposed plant from the 3 sources under consideration were computed, see Table 4. Overall economics and concluding comments
The incineration of sewage sludge also has problems of environmental acceptability. Environmental nuisances such as dust, grit and odour TABLE 4 Estimates of quantity of sludge and costs for disposal by incineration at 30°h d.s. ($7.8 Hong Kong=$l US)
Quantity of sludge (tonnes/day at 30% d.s. ) Option
Cost ($ millions) Capital Annual Replacement
Sha Tin
Sha Tin and Tai PO
127
179
Incineration plant at 200 t/day
incineratjon plant at 280 t/day
90 12.7 12
130 17.8 104
The analysis assumed that sludge is increasing at a steady rate, and the 1993 forecast figure is used as the ultimate quantity. Based on the assumptions mentioned, the relative costs of the various options are compared in terms of their cumulative net present values (NPV) using a 5% discount rate. The NPV’s of the options over a planning period of 20 years are summarized in Tables 5,6 and 7. The results support the following conclusions: ( 1) The cumulative NPV’s for disposal to Hong Kong waters are always lower than the respective cumulative NPV’s for disposal to international waters. This conclusion is rather obvious since the distance involved is much less, while other factors remain equal. It is also the least expensive means of sludge disposal among the various alternatives. (2) The cumulative NPV’s for incineration are
139 TABLE 5 Comparison
of options Sha Tin sewage treatment
Options
Disposal to Hong Kong waters
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
($7.8 Hong Kong=$l
US)
Disposal to international
waters
Disposal to tips
Incineration
2 no. 600 t vessels
1 no. 1,200 t vessel
108.60
217.90
186.50
118.40
234.40
1.00
2.01
1.72
1.09
2.16
TABLE 6 Comparison
of options Sha Tin and Tai PO sewage treatment
Options
Disposal to Hong Kong waters
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
($7.8 Hong Kong=$l
Disposal to international
US)
waters
Disposal to tips
Incineration
2 no. 600 t vessels
1 no. 1,200 t vessel
126.80
270.40
215.30
165.70
333.60
1.oo
2.13
1.70
1.31
2.63
TABLE 7 Comparison
of options Sha Tin, Tai PO and N.W. Kowloon ($7.8 Hong Kong=$l
Options
Cumulative NPV for 20 years ($ millions) Ratio of cumulative NPV
Disposal to Hong Kong waters
Disposal to international
US) Disposal to tips
waters
4 nos. 600 t vessel
2 nos. 600 t vessel and 1 no. 1,200 t vessel
2 nos. 1,200 t vessels
249.10
525.80
480.00
430.60
313.90
1.oo
2.11
1.93
1.73
1.26
always higher than the respective cumulative NPV’s for the other options. Hence, incineration is likely to be the most expensive option. (3) The cumulative NPV’s for disposal to landfill tips is the second best option, while the ratio of cumulative NPV of disposal to tips with respect to disposal to Hong Kong waters increases as the sludge quantity from same source increases. Hence disposal to tips is a limited option and should be used sparingly. A final decision on the method of disposal must also take into account environmental and ‘social
costs’, and not just the monetary costs. Many of the environmental effects, particularly those concerned with disposal at sea, are difficult to quantify and are not well understood. The social awareness of the general public must also be considered, since a method like incineration, which may not be environmentally damaging, will still ‘invite’ comments from the general public on its visual aspects. Dumping at sea will not be under the eye of the general public, and will be socially acceptable even if its environmental aspects may be more damaging.
140
The disposal of sludge to Hong Kong waters is sound from both operational and economic standpoints, at least in the short term. It is likely to be the least expensive among all options considered. Dumping in international waters is also acceptable at a slightly higher transportation cost. The disposal of sludge to international waters is also likely to be sound from a long term operational standpoint. It is likely to be the third least expensive among all options considered, but political advice and more site tests are required before this option can be chosen. The disposal of sludge to controlled tips is the second least expensive among all options considered. It is unlikely, if that option is chosen, that the solid waste disposal programme will be immune from the effects of reduced volume of tips for refuse. This is particularly suitable for sewage sludge from inland treatment plants. It is also suitable for agricultural sludge and alum sludge, and for the small amounts of sludge generated in the outlying islands. Incineration of sludge is likely to be the most expensive among all options considered. It should be adopted only where special circumstances justify its use, and hence only for small disposal quantities. It may also be desirable to maintain flexibility by having more than one type of major outlet, or even a combination of different outlets. The large majority of sewage sludge should, of course, be disposed at lowest cost in order to achieve overall efficiency. This should be done with due consideration of risk to human health, harm to living resources, damage to amenities, and restriction of normal activities of people. This case study has, in the main, been concerned with the establishment of general procedures to estimate the costs of the various options while maintaining an acceptable level of risk to the environment. The real life situation is, of course, dynamic, with costs constantly changing through technology improvements and the relaxation or tightening of other constraints. Sensitivity and other mathematical analysis using existing cost tigures may, therefore, turn out to be more of academic than of real interest. This study has only attempted to provide a general framework that can be used to quickly determine the economics involved at any point in time.
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