UK Developments in Sewage Sludge Processing

A considerable amount of research and development has been undertaken in the UK during the last 25–30 years in the field of sewage sludge processing. The research has been sponsored mainly by the water utilities with the objective of improving process efficiency and reliability and reducing costs. Government-funded research has focused mainly on the environmental aspects of sludge disposal to land and to the sea.
Finding ways of reducing sludge production has proved difficult, but one new method of sewage treatment has some advantages in this respect.
Arising from the research on sludge processing, (a) new methods of characterization of sludge have been developed, (b) an improved method for the design and operation of gravity thickeners has been successfully implemented, (c) advances have been made in the process of heated anaerobic digestion, and (d) an effective system for aerobic thermophilic digestion of sludge at small works has been established. A new method for measuring sludge filtrability – the PFT meter – has been developed, and the economics of flocculation have been improved by the introduction of a new type of mixer. An improved method for controlling the operation of filter presses is now available.
The problems of odour nuisance from sludge operations may now be controlled using biological systems to treat odorous air.     

Sewage Sludge Disposal in the UK: A New Challenge for the Next Twenty Years (Abridged)

The paper attempts to review current and future practices in the UK against a European background.
Sludge disposal has become a subject which is developing rapidly again and, since the presentation of this paper, there have been a number of developments, e.g. the adoption by the EC of its hazardous waste Directive in December 1991. Untreated sludge or sludge which is unsuitable for use in agriculture will be classified as a hazardous waste, subject to certain criteria. This makes the work on sludge classification even more crucial.     

Evaluation and Development of a Thermophilic Aerobic Digester at Castle Donington Sewage-Treatment Works

An investigation into the practicality of thermophilic aerobic digestion has been in progress at Castle Donington since 1989. The work has been carried out in a glassed steel insulated digester of 123 m³ capacity. Both auto-entraining and blower-assisted venturi systems have been evaluated.
Both aeration systems were able, under favourable conditions, to remove at least 50% of the sludge COD. It was found that the additional flexibility of the blower venturi combination was necessary to accommodate crude sludges varying from 2% to 12% dry solids.
Originally the plant was intended to operate at above 55C with a 10-day retention period. In practice retention periods at this temperature have been at least 15 days. The process has proved relatively susceptible to inhibition and requires the air supply to be closely matched to the load.
The revenue costs have been, at best, around 47.3/tDS. This is competitive with small-scale anaerobic digestion, but not with the large digestion centre approach widely used in the UK. Unless pasteurization becomes a requirement for agricultural disposal, thermophilic aerobic digestion is unlikely to find wide use in this country.     

Applications of Anaerobic Digestion for the Treatment of Industrial Wastewaters in Europe

For many decades, anaerobic digestion has been the most important technique of sewage sludge treatment at larger sewage-treatment works. Also, the low running costs of the process make it attractive for the treatment of strong industrial effluents. This paper reviews these advantages and discusses some disadvantages of the anaerobic treatment of industrial effluent. Operating and cost data are presented from Europe and from a detailed UK case study which supports the conclusion that anaerobic treatment will be the most cost-effective method of treatment of strong industrial effluents such as those from food and paper processing.     

Effectiveness of Secondary Digesters as a Pathogen Controller in Winter

A revision of the Regulations regarding the use of sewage sludge in agriculture has forced a change in emphasis in sludge treatment from stabilisation to achieving pathogen kill. Previous work carried out by United Utilities Service Delivery highlighted temperature as being a key factor in pathogen kill in mesophilic anaerobic digestion and a possible important factor in pathogen destruction in secondary digestion, which is normally carried out in large open tanks where temperature control is not possible. It was found that temperature is a key factor in the rate of pathogen de-activation under isothermal conditions. However, in the full-scale secondary digester trial under winter conditions it was found that there was a significant difference between the surface temperature and the bulk temperature within the sludge; at the surface the temperature decreased more quickly. There was no significant difference between the performance of the 'old'and 'new'design of concrete tank with respect to wall thickness, cooling rates and E. coli reduction rate. The glass-coated steel-tank design showed a marked increase in the cooling rate and, despite the low ambient temperatures and fluctuating sludge temperature, a minimum 1-log reduction in E. coli was achieved.     

Environmental Implications of Incineration and Other Advanced Sludge-Treatment Processes

In the UK, there is a strong trend towards more thermal processing of sludge with energy recovery or advanced treatment of sludge, in preparation for use in agriculture or other outlets. This has resulted from (a) the loss of the sea-disposal outlet and (b) pressure to improve the microbiological quality of recycled biosolids to land. In 1996–97, incineration accounted for 8% of sludge, and this figure is expected to increase to 21% by the year 2005. During this period, the annual production of sludge is expected to increase from 1.12 million tDS to 1.47 million tDS. More sludge will be treated by processes such as thermal drying, thermophilic digestion, prepasteurisation and mesophilic anaerobic digestion, lime treatment and other advanced options such as gasification. These developments apply across the EU and are likely to be driven forward by the impending revision of the 'sludge to land'Directive. The paper considers the environmental implications of these developments in terms of energy usage and recovery, air and water quality, greenhouse-gas emissions, effects on contaminants, and the quality of products or residues.     

Die-off of enteric bacterial pathogens during mesophilic anaerobic digestion

Conventionally treated sewage sludge may contain high concentrations of potentially pathogenic microorganisms and additional treatment is required to minimise the risks to health if it is to be recycled to agricultural land. Mesophilic anaerobic digestion (MAD) is the most widely used process in the UK for stabilising sludge prior to agricultural recycling, but little is known about the fate of a number of enteric pathogens as the sludge passes through the treatment processes. The aim of this study was to determine the efficiency of MAD in removing the bacterial enteric pathogens, Salmonella senftenberg, Listeria monocytogenes and Campylobacter jejuni which were added as a spike to the digester feedstock, together with the die-off of indigenous Escherichia coli already present in the sludge. The primary sludge digestion stage of MAD was found to achieve a log removal of 1.66 for E. coli, 2.23 for L. monocytogenes and 2.23 for S. senftenberg. However, the extent of die-off was a function of the numbers of pathogens in the feed and as these increased the log removal also increased. The numbers of C. jejuni were not affected by primary sludge digestion. Additional die-off was provided by secondary sludge digestion with log removals of 1.70 for E. coli, 2.10 for S. senftenberg and 0.36 for C. jejuni.     

Effect of ultrasound pretreatment in mesophilic and thermophilic anaerobic digestion with emphasis on naphthalene and pyrene removal

In many anaerobic digestion processes for the treatment of the sludge produced in wastewater treatment plants, the hydrolysis of the organic matter has been identified as the rate limiting step. This study is focused on the effect of ultrasonic pretreatment of raw sewage sludge before being fed to the mesophilic and the thermophilic anaerobic digestion. From particle size reduction, COD disintegration degree and biodegradability test, 11,000kJ/kg TS was estimated as the optimal specific energy in ultrasonic pretreatment. Moreover, the use of pretreated sludge improved significantly the COD removal efficiency and biogas production in lab-scale anaerobic digesters when compared with the performance without pretreatment, specially under mesophilic conditions. During ultrasonic pretreatment, the diffusion of polycyclic aromatic hydrocarbons (PAH) compounds to the aqueous phase was stated by a reduction in the pretreated sludge micropollutants content. With sonication, naphthalene was better removed than without this pretreatment, particularly in the mesophilic digester. However, pyrene removal remained at same efficiency level with and without ultrasonic pretreatment.     

The Use of High-Rate Nitrification for the Pretreatment of Ammoniacal Digested Sludge Liquors

Anaerobic sludge digestion reduces most of the organic nitrogen in raw sludge to ammonia. Where the digested sludge is dewatered, the ammonia in the liquors can represent up to 25% of the total ammonia load on the receiving wastewater-treatment plant. This can cause problems with compliance and also limit the throughput of the digestion plant. The paper describes how Anglian Water addressed this problem at Cliff Quay wastewater-treatment plant, Ipswich by installing the Amtreat^TM high-rate nitrifying process. The reasons for the necessity to reduce the ammonia load to the plant are presented and the rationale behind the selection of this solution is outlined. The Amtreat^TM plant is described and its performance is evaluated. The Cliff Quay site is the first application of this process to a municipal wastewater-treatment plant in the UK, although there are several installations operating with industrial effluents.     

THE EFFECTS OF IMPROVING SEWAGE SLUDGE DIGESTION

A model was developed and tested to investigate the effects of various parameters on the processing costs of anaerobic digestion. The model was based on empirical data collected from full-scale plants which varied in size from 35,000 to 900,000 population equivalents. (Equivalent to loading rates of 0.5 to 2.7 kg VS/m³/d.) In spite of different operating regimes and conditions of the plants analysed, various relationships were discovered between: sludge quality and volatile solids destruction; sludge quality and biogas yield; and, effluent volatile solids and dewatering. Where correlations existed, these were used to make predictions on operating costs of the digestion and dewatering plant based on sensitivity analysis. The results of the model were used to make recommendations on how to optimise the operation of an anaerobic digestion plant.     

Treatment and Disposal of Municipal Sludge in The Netherlands

This paper reviews the sludge treatment and disposal operation in The Netherlands where the total volume of sludge is increasing and the sales potential is falling due to stringent quality requirements. The sludge can no longer be recycled to agriculture and has to be dumped on land. The techniques available for this purpose make sludge disposal much more expensive.
A meticulous analysis of the entire sludge chain seems to be necessary to control the costs and minimize the environmental impact, and a start has recently been made on the formulation of ideas in this context.     

Developments in the Thermal Drying of Sewage Sludge

The application of heat to dried sewage sludge has been practised by the water industry for many years. Early technologies were effective but their utilization of energy was not so efficient. Other industries have faced up to this problem and, as a result, have developed thermal-drying techniques which are more efficient and therefore more cost effective. In recent years, the transfer of this technology to sewage-sludge drying has been taking place, and some companies have designed dryers specifically for this purpose. Commercial and legal pressures have also focused attention on the final product and its use in agriculture, horticulture, land reclamation, landfill, incineration, and as a substitute for fuel.
This paper reviews the work which has been undertaken to determine the changes which occur as sewage sludge is dried, and discusses the technology which is available at present. Factors influencing capital and operating costs and product choice are also included.     

Sludge dewatering in a conventional plant with phosphorus removal-I: Analysis of additional costs

The aim of this research is to assess the differences in quantity and quality of sludges produced in a conventional activated sludge plant, modified for phosphorus chemical removal. Tests were carried out at the Cesenatico sewage treatment plant, where in summer phosphorus is removed simultaneously to the biological process with the aid of aluminium or ferrous sulphate. Characteristics of the sludge produced with or without phosphorus removal were compared. The sludge was then dewatered by a centrifuge and a filter-press on pilot scale and many tests were carried out under different operating conditions.In this part, the plant's most important performances are discussed with regard to phosphorus removal efficiency, sludge production, characteristics and dewaterability. The experiments showed that phosphorus could be removed up to residual concentration of 1.6 mg 1^?1 without detrimental effects on the biological process, with an increase of sludge production of 21 and 36% using aluminium or ferrous sulphate respectively. On the basis of experimental results a costs analysis was carried out to assess the costs for phosphorus removal, including the additional ones for sludge conditioning, dewatering and cake transport to landfill. It results that additional costs vary from 3290 to 6380 Lit per capita per year (29–54%). The higher costs refer to smaller plants (50,000 inhabitants), in which aluminium sulphate is used and sludge dewatered by centrifuge. The use of ferrous rather than aluminium sulphate allows savings of 1300–1600 Lit per capita per year.     

The cost of a large-scale hollow fibre MBR

A cost sensitivity analysis was carried out for a full-scale hollow fibre membrane bioreactor to quantify the effect of design choices and operational parameters on cost. Different options were subjected to a long term dynamic influent profile and evaluated using ASM1 for effluent quality, aeration requirements and sludge production. The results were used to calculate a net present value (NPV), incorporating both capital expenditure (capex), based on costs obtained from equipment manufacturers and full-scale plants, and operating expenditure (opex), accounting for energy demand, sludge production and chemical cleaning costs.Results show that the amount of contingency built in to cope with changes in feedwater flow has a large impact on NPV. Deviation from a constant daily flow increases NPV as mean plant utilisation decreases. Conversely, adding a buffer tank reduces NPV, since less membrane surface is required when average plant utilisation increases. Membrane cost and lifetime is decisive in determining NPV: an increased membrane replacement interval from 5 to 10 years reduces NPV by 19%. Operation at higher SRT increases the NPV, since the reduced costs for sludge treatment are offset by correspondingly higher aeration costs at higher MLSS levels, though the analysis is very sensitive to sludge treatment costs. A higher sustainable flux demands greater membrane aeration, but the subsequent opex increase is offset by the reduced membrane area and the corresponding lower capex.     

Ultrasonic Sludge Pretreatment for Enhanced Sludge Digestion

During recent decades, the anaerobic digestion process has been extensively studied and various methods for process enhancement have been explored. These methods include heat treatment, alkali addition, phase separation and membrane enhancement. In general, whilst technically feasible, the methods have not proved to be economically competitive.
The pretreatment of sludges with high-power ultrasound has been investigated as part of a twelve-month research project funded by six UK water utilities. Cell lysis and particle-size reduction (caused by ultrasonic cavitation) are thought to be the key parameters through which the digestion process is enhanced. A series of laboratory-scale anaerobic digesters has been operated and significant increases in biogas yield have been noted following ultrasonication. Experiments have been completed with a variety of ultrasonic devices (of different geometries and construction materials) and sludge types (e.g. primary, secondary and co-settled). Current experiments are investigating methods through which the technology can be successfully scaled-up and applied on a full-scale plant.     

The Impact of EU and UK Environmental Pressures on the Future of Sludge Treatment and Disposal

Current and future production of sludge in the UK is estimated, and it is predicted that recycling to agricultural land and incineration (with energy recovery) will be the major disposal options for sludge in the future. Environmental pressures on sludge recycling to land may lead to restrictions on applications in terms of nitrogen content and more stringent limits for metals in soils. Attention to sludge quality and the development of quality management practices in utilization or disposal operations will help to minimize environmental concerns and facilitate sludge disposal to all outlets. Focal points for quality assurance in sludge recycling operations are listed, and environmental pressures on sludge use in agriculture are discussed in detail. Use in agriculture involves recycling a secondary resource, and calling the resource 'biosolids' is part of promoting this excellent example of good environmental management.     

The Impact of the EC Urban Waste Water Treatment Directive

During the last two years significant policy and legislative developments have taken place in the UK and in the EC in respect of sewage treatment and disposal and the dumping of sewage sludge at sea. Details of the UK Government's sewage-treatment initiative and its decision to stop the dumping of sewage sludge at sea by 1998 are described. An outline is given of the provisions of the EC urban waste water treatment Directive, in particular those provisions concerned with sewage treatment, trade effluent discharges from prescribed industrial sectors, and the termination of dumping of sewage sludge at sea. The implications of these developments for Scottish sewerage authorities are summarized and provisional estimated costs of meeting these new obligations are given. The opportunities available to contractors and equipment manufacturers are highlighted.     

Preliminary Results of the Anaerobic Biotreatability of the Effluent from the Wet-Air Oxidation of Sewage Sludge

Wet air oxidation and other thermal processes can be used on all sludges, whether they are raw, mixed or digested. The process sterilizes the sludge and gives a considerable reduction in volume. However, apart from the high capital costs which are associated with this technique, one of its principal disadvantages is the high strength of the (effluent) liquor.
This study investigated the feasibility of treating the liquor using an anaerobic digester, which resulted in a good reduction in BOD and a reasonable reduction in COD. The liquor was still strong, but could be returned to the inlet works of a sewage-treatment plant without causing significant organic overloading.     

Developments in Sewage Sludge Incineration

W ithin THE LAST few years there has been a resurgence of UK interest in incineration as a sludge disposal option. However, the last plant installed in the UK represented the technology and design thinking of the 1970s and there have been significant developments since that time. These are reviewed in this paper.
Especial attention is given to changes in legislation and attitudes concerning the environmental impact of incineration, and the effects on process selection. The wide range of options for the achievement of autothermic operation are itemized and a brief review of costs given.
It is considered likely that incineration will have an increased role in the future, though continuing to be rather specialized. Keeping an open mind on equipment selection is urged, as is the conduct of an organized programme of work to establish process design information before going out to tender.     

THE EFFECTS OF ULTRASOUND ON SLUDGE DIGESTION

Ultrasound treatment, which involves the introduction of high-intensity sound waves into a sludge medium, is one of several technologies which promote hydrolysis during sludge treatment. It has become well-established with numerous full-scale plants in Europe operating for several years. The basic principle involves the release of extra-cellular material which then catalyses biological reactions and improves bacterial kinetics, resulting in lower sludge quantities and (in the case of anaerobic digestion) increased biogas production. Its use is most suited to plants containing large quantities of refractory material and/or cellular matter, such as waste activated sludge. This paper uses the data from several full-scale part-stream ultrasound plants and discusses the influence of the technology on numerous operating conditions, especially with respect to improved digestion and enhanced biogas production.