Iwa international Specialist Conference

Author: Oddvar Tornes, IVAR IKS - Norway

Co- authors: Alan Whipps, Pell Frischmann Ltd - UK
The Regional Wastewater Treatment Plant of North Jæren (IVAR IKS) serves the Stavanger conurbation with a population equivalence of 240,000. The site was the first in Norway to operate a thermal drying and a pelletising plant for municipal sludge. The drying plant was commissioned in January 1993 and achieved operational status in July 1994. The pelletising plant was installed as an integral part of the drying facility in order to ensure a product that is easy to handle, store and suitable for transport and spreading on land.
The dryer is an indirect disc dryer, which utilises the biogas production in anaerobic digesters as heat. The biogas is used as fuel in a gas-burner, which produces steam for the dryer and hot water for the central heating system. The heat recovery from the drying process is sufficient to meet the heat requirements for all buildings and the anaerobic sludge digestion processes.
In general, thermal drying is considered a costly and complex process, requiring careful attention to safety aspects like risks of self-combustion, fires and dust explosions. A number of operational problems of various nature have been reported. In some cases, dust explosions have resulted in severe damages and personal injuries. It appears that many operational problems are linked to poor consideration of operating procedures and lack of basic knowledge of the sludge characteristics that influence thermal drying processes.
Since the start up, IVAR has encountered many of the operational problems typically associated with thermal drying processes, including smouldering, odour problems, condensation and dust accumulation in the conveying systems, wear problems, heat losses in the steam return condensate and difficulties in achieving fully automatic operation. Considerable modifications have been made and lessons learnt resulting in the plant being one of the few thermal drying facilities in Norway which continues to operates successfully.
A feasibility study conducted in 1999 demonstrated the profitability of expanding the plant with a new parallel line within 2002. The new plant is currently under construction and commissioning is scheduled for April 2003. Optimisation of the operational costs and providing safe operation were key requirements both in the procurement strategy and the detail design. Implementation of IVAR’s and international operating experiences and undertaking risk assessment has formed the basis for the detail design for a complete new drying plant. The paper will present a review of the following topics and their influence on the detail design.

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  HAZOP ( Hazard and Operability) study of the thermal drying plant and associated equipment
  
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  Conservation of energy, condensing/heat recovery systems and heat/energy balance for the complete sludge treatment.
  
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  Capital and operating costs.
  
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  Operational procedures and automation.
  

86.

Plate and Frame Press Optimisation for Potable Water Treatment Derived Sludges

Martin R. Tillotson^*, David R. Dixon^**, Peter J. Harbour^**, Peter Hillis^†, Peter J. Scales^**, and Anthony Stickland^**

^*Yorkshire Water Services Limited, Western House, Western Way, Bradford BD6 2LZ, United Kingdom.

^†United Utilities PLC, Lingley Mere, Lingley Green Avenue, Warrington WA5 3LP, United Kingdom.

^**Department of Chemical Engineering, University of Melbourne, Victoria, Australia 3010.

Abstract

Capillary suction time and specific resistance to filtration measurements are commonly used for the characterisation of sludges and the prediction of efficient operation of dewatering processes such as filtration and centrifugation. Whilst useful in predicting trends, these methods rely on non-fundamental properties of the sludge and do not lend themselves to the quantitative design and optimisation of dewatering devices. Recent work by the group has led to the development of a stepped pressure filtration technique capable of rapid measurement of permeability and compressibility of sludges. The work has shown that a single volume fraction dependent parameter, termed the solids diffusivity, calculated from the permeability and compressibility, is able to fully describe sludge filtration behaviour. The information allows unequivocal comparison of sludges derived from different raw water sources, and highlights the role of flocculants and the importance of their selection. Comparative data is presented from a range of plants from both the UK and Australia.

A further goal of this work is to use solids diffusivity data as one of the inputs to first principles models useful for the prediction of performance of solid-liquid separation devices. An early achievement has been the development of a fully validated plate and frame model able to predict filtration behaviour in both fill-only and membrane inflation type press units. Several case studies are presented in which this work has been successfully applied to the design of new sludge processing plant and in increasing the operational efficiency at existing works. The next step in this work is the application of these techniques to the processing of biosludges and other dewatering devices.

THE FUNDAMENTALS OF SLUDGE CHARACTERISATION AND FILTRATION

87

P J Scales*, N J Anderson**, D R Dixon*, P J Harbour**, R G de Kretser* and A D Stickland*

* PFPC, Department of Chemical Engineering, University of Melbourne, AUSTRALIA

** CMIT, CSIRO, Highett, AUSTRALIA

ABSTRACT

A key drawback in the design and optimisation of industrial filters has been the varying degrees of empiricism in most design and prediction methodologies. The linearised filtration theory presented by Landman and White utilises two fundamental material parameters to quantify the dewaterability of a material in terms of its compressibility and permeability. The parameters, the hindered settling function, r(), and the compressive yield stress, P_y() are characteristic of the material and its properties eg. surface chemistry, preconditioning.

Until recently, rapid measurement of P_y() and r() was not possible, however, the development of a multi-pressure filtration technique has enabled rapid measurement of the parameters over a wide range of pressure conditions. The theoretical and experimental framework developed now enables material characterisation and performance testing of various conditioning methods with the advantage that the parameters used to evaluate dewatering efficiency are directly applicable to full scale modelling of filtration and other dewatering unit operations.
The paper to be presented will outline the parameters and their characterisation along with their use in filtration modelling for a range of waste materials. Implications of the filtration theory for modelling and optimisation of filtration cycle times and chemical conditioning will be discussed in the context of both water and sewage treatment plants. Validation of the model predictions against real filtration data for a number of materials including water treatment plant residuals and biosolids and a range of filtration conditions will be presented.
CHARACTERIZATION AND EVALUATION OF POTENTIAL REUSE OPTIONS FOR WASTEWATER SLUDGE AND COMBINED SEWER SYSTEM SEDIMENTS IN MEXICO

B
88
. Jiménez, J. M. Méndez, J. A. Barrios, and G. Salgado

Institute of Engineering, National University of Mexico (UNAM)

Apartado Postal 70-472, Coyoacan 04510, Mexico, D. F. Mexico.

In Mexico City, the combined sewer system transports both rainfall and wastewater and present several phenomena that include sedimentation of solids within the system, but additionally, it carries the sludge produced by 24 wastewater treatment plants. The system is conformed by a primary network (0.76 to 3.05 m Ø pipes); secondary network (0.30 to 0.61 m Ø pipes) and a drainage system (deep network, open channels and flood regulation structures -mainly dams and lagoons-) with a combined capacity of 11.2 Mm³. The annual generation of sediments by the whole system is estimated in about 2.8 Mm³, which includes an estimate of 0.42 Mm³ of sludge. As a result, the total capacity for transporting water is reduced considerably, making necessary to extract yearly an approximate 0.85 Mm³ of those materials and to send them to final disposal sites. On this regard, the main disposal site is the Bordo Poniente landfill located in the northeast part of the City that has an expected life of 2 or 3 years. This situation has forced the local Government to seek for potential reuse options for these materials. As part of this effort this project evaluated the quality of sediments from 5 dams, 4 regulation lagoons, 2 open channels, and 3 transfer stations that collect the sediments extracted with Vactor® trucks. Analyses performed included corrosivity, reactivity, explosivity, toxicity and inflammability tests to determine if the materials are hazardous according to the Mexican regulation (NOM-052-ECOL-1993). Also, conductivity, pH, humidity, metals, organic matter total petroleum hydrocarbons, BTEX compounds, ammonia and total nitrogen, total phosphorous, faecal coliforms, Salmonella spp. and helminth ova were evaluated. So far the results indicate that there is an important presence of lead in some sites, particularly in 33% of the sampled regulation structures and 66% of the transfer stations. Additionally, total petroleum hydrocarbons or BTEX were detected in 12 sites from the 14 sampled. Moreover, none of the sites met the limits for Salmonella spp. according to the proposed biosolids regulation (< 300 NMP/g ST; NOM-004-ECOL-2001). Based on the results, a classification was established to determine the degree of contamination of the sediments as well as the required treatment to allow their potential reuse. Also, options for reuse were determined based on the characteristics of the materials and particular needs of the population, considering the lack of information on this topic and their particular characteristics. One of the potential reuse options is to use the sediments as leveling material for flooded areas where farming has been limited due to the yearly loss of at least 450 ha of ground that sinks about 0.5 m every year. Final recommendations include developing erosion control programs to reduce the amount of material that enters the system as well as the adequate management of wastewater sludge to reduce its input to the drainage. These measurements should be followed with a discharge control program as well as an intensive monitoring of the quality of the sediments and sludge.

Bioleaching and chemical leaching of heavy metals from anaerobically digested sludge

89

M. M. Marchioretto, N. T. P. Hien, H. Bruning and W. H. Rulkens

Sub-department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands. Phone: [phone]. Fax: [phone]. E-mail: [e-mail]

Introduction

The conventional activated sludge system is a technology globally used for treating domestic sewage. It has, however, the drawback of excessive sludge production. Usually this produced sludge is stabilized in anaerobic digesters, dehydrated and disposed in landfills, resulting in grave problems due to the shortage of disposal capacity and leaching of heavy metals to the soil, surface water and underground water. In the Brazilian State of São Paulo, which is known for its high level of urbanization and industrialization, the issue of the final disposal of the wastewater sludge is even more serious because of the high heavy metal content of the sludge. Santos and Tsutiya (1997) estimate that the total sludge production in year 2005 will be around 615 dry metric tons per day in São Paulo State.

Without a pre-treatment focused on the dissolving heavy metals, a physical separation process cannot further remove these metals. Chemical leaching with acids or complexing agents is an efficient option to promote the solubilization of the metals. This alternative, however, is extremely expensive due to the large consumption of chemical agents. Biological leaching (bioleaching) can be a feasible and sustainable alternative to reduce the costs of the acidification method. It is based on a microbial production of sulfuric acid and leaching of metals. According to Tichy et al. (1998), to achieve sufficiently low pH, proper amount of reduced sulfur or ferrous iron must be present in the material to be treated. When this is not the case, additional substrate has to be introduced.

The aim of the present research is to evaluate the efficiency of bioleaching comparable with chemical leaching to extract heavy metals present (Cr, Cu, Pb, Zn) in the anaerobically digested sludge. For that, chemical leaching with sulfuric acid and bioleaching using elemental sulfur and ferrous iron as substrates were applied. The presence of indigenous bacteria in the sludge and their ability to solubilize heavy metals were considered.

Materials and Methods

Anaerobically digested sludge

The sludge applied in this research is originated from an anaerobic digester of a wastewater treatment plant located in Schijndel, The Netherlands. Table 1 shows some characteristics of the sludge, the percentage of metals content in the liquid part (supernatant) of the centrifuged sludge and the Dutch Standard for metals disposal on agriculture soils - BOOM (SDU, 1991).

Table 1. Characteristics of the anaerobically digested sludge

Parameter	Dry Matter (DM)	Organic Matter	Heavy Metal Content (mg/kg DM)
	(g/L )	(% DM)	Cr	Cu	Pb	Zn
Total sludge (TS)	25	60	300-500	700-900	150-200	1500-2000
BOOM	-	-	75	75	100	300
Supernatant	-	-	3.8% TS	4.0% TS	3.0% TS	4.0% TS

Chemical leaching experiments

Ten 1-L glass bottles were filled with 400-ml of sludge, continuously agitated (125 rpm) at 30C and simultaneously exposed to aeration for 24 hours before and during the acidification. After previous aeration, acidification with sulfuric acid was applied for each bottle to achieve initial pH values of 5, 3, 2, and 1. Samples in duplicates of 15-ml sludge were collected at acidification times of 0, 5 hours, 7 hours, 9 hours, 1 day, 2 days, 4 days, and 5 days. The redox potential and the pH were measured every day and when the sample collections were performed. The best pH condition for metal solubilization was repeated with hydrochloric acid, in order to compare with sulfuric acid. So far, aeration followed by acidification with hydrochloric acid has turned out to be the most effective way to solubilize metals from the present sludge (Marchioretto et al., 2001, and Marchioretto et al., 2002). The experiments were done in duplicates.

Bioleaching experiments

All the sludge bottles were continuously agitated (125 rpm) at 30C and simultaneously exposed to aeration for 24 hours before and during the bioleaching step, which ran for 15 days. Carbon dioxide was applied as carbon source for the autotrophic bacterial growth. The pH was measured every day and samples in duplicate of 15-ml sludge were collected only when the pH value was stable (pH around 3-4, after 9 days).

- With addition of elemental sulfur (S, S/Acid):

Four 1-L glass bottles were filled with 400 ml of sludge and amended with elemental sulfur microbiologically produced by a pilot plant treating sulfide-rich wastewater in Eerbeek, The Netherlands. The pH of two bottles was adjusted to 5 by sulfuric acid and the remaining bottles were preserved with the natural pH.