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Dewatering of sludge
Water Research Pergamon Press 1972. Vol. 6, pp. 527-529. Printed in Great Britain
DEWATERING OF SLUDGE W. H. MEREDITH
Howard Humphrey & Sons, Consulting Engineers, WestminsterHouse, West Street, Epsom, Surrey, England A DECISION on the mechanical dewatering of raw and digested sludges calls for an initial appraisal of the outline economics related to the budgetary costs of capital construction and estimate of operating costs. The latter at times can be the subject of material revision when the plant is installed and operated, and demands a firm performance undertaking from the plant contractor at tender stage. Whilst pilot experimentation will give an indication of the feasibility of the particular sludge conditioning and dewatering arrangements, many aspects of problems associated with continuous 24 h a day operation are revealed only with full scale operation. My particular experience with thermal conditioning of sludge at a works in England demonstrates this. The plant concerned is rated to process some 2,000,000 imperial gal (9000 m 3 say) of digested sludge per 7-day week, and the initial outline economics indicated that heat conditioning and pressing was the most favourable arrangement. The relative basic figures were:
Heat Chemical conditioning conditioning and pressing and pressing (£) (£) (£1oo = $240) Labour Fuel oil Power costs Chemicals Maintenance Annual costs Capital costs--M/c. --Civil
5000 2500 1200 -1200
4000 -1500 9000 600
£9900
£15,100
80,000 90,000
80,000 58,000
£170,000
£138,000
In the event, the degree of plant supervision and maintenance required has proved materially in excess for the heat conditioning plant to an extent which absorbs the originally assessed differences in operational costs. An up-to-date assessment of the comparative costings is as follows: 527
528
W . H . MEREDITH
The beat conditioning plant as installed was geared to a duplication of capacity for whicb operating costs per annum are assessed on the basis of processing 18 × 106 gal yr-1 (820,000 m 3 say) digested sludge at 97 % water content.
Heat Chemical conditioning conditioning and pressing and pressing (£) (£) Labour Fuel equivalent Power costs Odour control Chemicals Maintenance
Labour Power Incidental Maintenance Odour control Fuel equivalent
9000 3600 2000 2000 -3000
6000
£19,600
£18,000
2000 9000 1000
£
£'s per 1000 gal
9000 2000 1000 3000 2000 4000
0.5 0.11 0.05 O.16 0-11 0.22
£21,000
£1-15
Capital costs are equivalent to £1.50 per 1000 gal (4.5 m 3 say) with handling to dump on site at £0.15. This gives an inclusive cost o f some £2.80 per 1000 gal. A similar rated plant using chemical conditioning shows an overall costing at some £3 per 1000 gal. This plant is without the attendant maintenance and odour problem o f the heat conditioning plant. Arising from these costings, there is no doubt we have a much clearer understanding o f the relative merits of chemical and thermal conditioning processes with the balance definitely in favour of the former. The performance o f the thermal conditioning plant is materially affected by the progressive clogging of the heat exchanger tubes particularly at the primary section o f the arrangement. The incidence of incremental clogging is materially affected by the water content o f the digested sludge stock feed. Experience over some 12 months indicates that the optimum water content is at 97 per cent for the digested sludge. At this level an effective continuous operation over 21-28 days can be achieved. At 96-96.5 per cent the period is reduced to less than 4 days. This experience emphasizes the mx:cssity for the contractual performance to be linked to a definite range o f sludge water content.
Dewatering of Sludge
529
The main design factor arising from the operational difficulties is the active consideration of an alternative arrangement for the primary tubes, at which the conditioned sludge gives up heat to the feed stock sludge. This primary section consists of a 2.5-in. bore inner tube set in a 4 in. bore tube. The main incremental clogging occurs at this 4 in. bore and the indications are that a sludge to water alternative arrangement at this primary section would naturally reduce the incidence of clogging, and of course, effectively double the necessary length of tubes. The odour problem at this particular plant is mainly related to the separated decantrate waters from the heat conditioned sludge and the press filtrate which follows. A fall off in performance at the primary tubes leads to a rising temperature of the decantrate waters separated out from the conditioned sludges before pressing. If this temperature exceeds a value of 43-46°C local aerial nuisance is set up and working conditions at the press house become more difficult despite the controlled ventilation system as installed. The overall performance of the plant is very adequate with good sludge-press cakes at 50 per cent water content at pressing periods of 3-4 h coupled to some 20 rain conditioning period at not less than 188°C. A fall in this conditioning temperature calls for longer press periods and acceptance of wetter press cakes---60 per cent water content. It is hoped that these few introductory notes which cover some salient factors of observed plant operation will give some lead to the adjustment of detailed planning and design in the field of sludge conditioning and dewatering.
DEWATERING OF SLUDGE W. H. MEREDITH
Howard Humphrey & Sons, Consulting Engineers, WestminsterHouse, West Street, Epsom, Surrey, England A DECISION on the mechanical dewatering of raw and digested sludges calls for an initial appraisal of the outline economics related to the budgetary costs of capital construction and estimate of operating costs. The latter at times can be the subject of material revision when the plant is installed and operated, and demands a firm performance undertaking from the plant contractor at tender stage. Whilst pilot experimentation will give an indication of the feasibility of the particular sludge conditioning and dewatering arrangements, many aspects of problems associated with continuous 24 h a day operation are revealed only with full scale operation. My particular experience with thermal conditioning of sludge at a works in England demonstrates this. The plant concerned is rated to process some 2,000,000 imperial gal (9000 m 3 say) of digested sludge per 7-day week, and the initial outline economics indicated that heat conditioning and pressing was the most favourable arrangement. The relative basic figures were:
Heat Chemical conditioning conditioning and pressing and pressing (£) (£) (£1oo = $240) Labour Fuel oil Power costs Chemicals Maintenance Annual costs Capital costs--M/c. --Civil
5000 2500 1200 -1200
4000 -1500 9000 600
£9900
£15,100
80,000 90,000
80,000 58,000
£170,000
£138,000
In the event, the degree of plant supervision and maintenance required has proved materially in excess for the heat conditioning plant to an extent which absorbs the originally assessed differences in operational costs. An up-to-date assessment of the comparative costings is as follows: 527
528
W . H . MEREDITH
The beat conditioning plant as installed was geared to a duplication of capacity for whicb operating costs per annum are assessed on the basis of processing 18 × 106 gal yr-1 (820,000 m 3 say) digested sludge at 97 % water content.
Heat Chemical conditioning conditioning and pressing and pressing (£) (£) Labour Fuel equivalent Power costs Odour control Chemicals Maintenance
Labour Power Incidental Maintenance Odour control Fuel equivalent
9000 3600 2000 2000 -3000
6000
£19,600
£18,000
2000 9000 1000
£
£'s per 1000 gal
9000 2000 1000 3000 2000 4000
0.5 0.11 0.05 O.16 0-11 0.22
£21,000
£1-15
Capital costs are equivalent to £1.50 per 1000 gal (4.5 m 3 say) with handling to dump on site at £0.15. This gives an inclusive cost o f some £2.80 per 1000 gal. A similar rated plant using chemical conditioning shows an overall costing at some £3 per 1000 gal. This plant is without the attendant maintenance and odour problem o f the heat conditioning plant. Arising from these costings, there is no doubt we have a much clearer understanding o f the relative merits of chemical and thermal conditioning processes with the balance definitely in favour of the former. The performance o f the thermal conditioning plant is materially affected by the progressive clogging of the heat exchanger tubes particularly at the primary section o f the arrangement. The incidence of incremental clogging is materially affected by the water content o f the digested sludge stock feed. Experience over some 12 months indicates that the optimum water content is at 97 per cent for the digested sludge. At this level an effective continuous operation over 21-28 days can be achieved. At 96-96.5 per cent the period is reduced to less than 4 days. This experience emphasizes the mx:cssity for the contractual performance to be linked to a definite range o f sludge water content.
Dewatering of Sludge
529
The main design factor arising from the operational difficulties is the active consideration of an alternative arrangement for the primary tubes, at which the conditioned sludge gives up heat to the feed stock sludge. This primary section consists of a 2.5-in. bore inner tube set in a 4 in. bore tube. The main incremental clogging occurs at this 4 in. bore and the indications are that a sludge to water alternative arrangement at this primary section would naturally reduce the incidence of clogging, and of course, effectively double the necessary length of tubes. The odour problem at this particular plant is mainly related to the separated decantrate waters from the heat conditioned sludge and the press filtrate which follows. A fall off in performance at the primary tubes leads to a rising temperature of the decantrate waters separated out from the conditioned sludges before pressing. If this temperature exceeds a value of 43-46°C local aerial nuisance is set up and working conditions at the press house become more difficult despite the controlled ventilation system as installed. The overall performance of the plant is very adequate with good sludge-press cakes at 50 per cent water content at pressing periods of 3-4 h coupled to some 20 rain conditioning period at not less than 188°C. A fall in this conditioning temperature calls for longer press periods and acceptance of wetter press cakes---60 per cent water content. It is hoped that these few introductory notes which cover some salient factors of observed plant operation will give some lead to the adjustment of detailed planning and design in the field of sludge conditioning and dewatering.