Iwa international Specialist Conference

Abstract

New regulations or recommendations encourage WWTP manufacturers to consider processes which not only eliminate the nuisance caused by biosolids but also enable the recovery of energy and/or recycling of the material components. This paper highlights two thermal oxidation processes meeting these new demands:

The first one is the incineration, in a dedicated fluidised bed sludge incinerator, working @ 850°C in gaseous phase, which is well suited to deliver efficient heat power and possibly CHP (Combined Heat and Power) electricity generation for large plants. Typically, considering dewatered sludge @ 28% dry solid, 75% volatile content, up to 40% of the energy input can be recovered without extra fuel consumption during steady state. Moreover, appropriate flue-gas treatment is provided which minimises pollutant emissions, complying with the most stringent regulations for particulates, acidic compounds, heavy metals, dioxins and other parameters. In this way, the environmental impact of sludge incineration is dramatically reduced and can be even more favourable than land application.

The second solution is Wet Air Oxidation (WAO), which thermally oxidises biosolids in the liquid phase (245°C, 4.5 MPa) and consequently delivers clean, contained by-products (neither flue gas, nor flying ash). Thermal self sustainability is met as soon as the COD concentration reaches 50 kg/m³ in the inlet sludge. Typically, over 80% of COD is destroyed, the remaining 20% being mainly soluble highly biodegradable molecules such as acetate or propionate. In this way, the treated liquid effluent from this WO process provides a readily available source of free carbon to the wastewater treatment facility and consequently improves the C/N ratio for the activated sludge process. In some cases of denitrification, extra carbon source such as methanol can be saved. The mineral residue retains heavy metals in a non leachable form. In addition, pressurised hot water can be produced from WAO.

For each process, the mineral by-products can be upgraded into a reusable material such as a raw material for the cement industry or road construction. Fossil energy consumption and reduction of greenhouse gases (CO₂, CH₄ and N₂O) are considered in both cases and compared with land application. Ultimately, we show that optimisation of energy recovery and minimisation of nuisances generated from biosolid treatments are achieved by combinations of processes, such as a digestion upstream the thermal oxidation step. Such arrangement offers a stand-by disposal route improving plant availability and reduces both total footprint and final residue to dispose.

55.

Direct Energy Recovery from Primary and Secondary Sludges

by Supercritical Water Oxidation

by

Michael Modell and Jefferson Tester

Laboratory for Energy and the Environment

Massachusetts Institute of Technology

Cambridge, MA 02138 USA

Abstract

Supercritical water oxidation (SCWO) is considered to be a total solution for destruction of sewage sludges. It is a process for oxidizing organic materials very effectively and virtually completely to carbon dioxide and water. The temperature regime is 500 to 650^oC at pressures in the range of 25 MPa. Heavy metals are oxidized to the corresponding oxides and recovered as a benign and stabilized solid, along with the sand and clay that is present in the feed. Organic chlorides and sulfur compounds are oxidized to the corresponding acids, which are automatically scrubbed out of the effluent as it is cooled to room temperature. The aqueous effluent is a brine containing these acids along with salts that are present in the feed. Organic nitrogen is converted to molecular nitrogen, which exits in the gaseous effluent, along with carbon dioxide and excess oxygen.

Effective oxidation at the mild temperatures of SCWO is made possible by high pressure and the presence of water as the reaction medium. Water above 374^oC and 22 MPa is a supercritical fluid. In that state, supercritical water (SCW) becomes a superb solvent for organic materials as well as gases. In addition, SCW reacts with organics and reforms them to small molecules - without the formation of char. These small molecules are readily oxidized if oxygen is present with the organic-SCW mixture.

The technology has been under development for twenty years. Several full-scale systems have been or are being built. The major obstacle to commercialization has been developing reactors that are not clogged by inorganic solid deposits. That problem has been solved by using tubular reactors with fluid velocities that are high enough to keep solids in suspension.

The technology has gained a reputation as being expensive. Early applications using vessel reactors with rare earth liners for destruction of wastes from chemical weapons were expensive. Most recently, system designs have been created that reduce the cost of processing sewage sludges below that of incineration. Specifically, configurations have been developed that require no excess fuel for aqueous wastes with a minimum heating value of 500 kJ/kg. For wastes with 1,700 kJ/kg, 70% of the heating value of the waste can be recovered as high- pressure steam. A sewage sludge with 10 wt-% solids has about 1,700 kJ/kg heat content. Sewage sludges can readily be concentrated to 10 wt-% solids by centrifugation, without addition of coagulants.

In addition to production of high-pressure steam, liquid carbon dioxide of high purity can be recovered from the gaseous effluent by isenthalpic expansion. In this manner, excess oxygen can also be recovered for recycle. The net effect is to reduce the stack to a harmless vent with minimal flow rate of a clean gas.

Complete simulations have been developed for these new process designs. Physical property models have been developed that accurately simulate the thermodynamic properties of sub- and supercritical water in mixtures with O₂, N₂, CO₂, and organics.

The scenario of siting a SCWO unit for sludge disposal at a power plant, where the full potential for steam credit can be realized, will be discussed. Material and energy balances were generated from Aspen models of the entire process. Capital and operating cost estimates will be given for sewage sludge treatment. This scenario of direct recovery of energy from sludges has inherent benefits compared to other gasification or liquefaction options.

56.
RECYCLING OF SLUDGE WITH THE AQUA RECI-PROCESS
Kjell Stendahl, Feralco AB, Industrigatan 126, SE-252 32 Helsingborg, Sweden

phone [phone], fax [phone], e-mail: [e-mail]

Abstract

Sludge produced at our modern sewage treatment plants is a growing problem. Rigorous restrictions are being enforced on a European level. Even if the sludge could be considered as a resource, for example as a fertiliser on farmland due to its content of plant nutrients, the potential of hazardous components is limiting such solutions for sludge disposal. Another regulation coming up inside the EU in 2005 is a ban on disposing organic material on landfills.

In order to solve the sludge issue, it is important to look for new alternative methods. One feasible new method is “supercritical water oxidation” which makes it possible to completely decompose organics, including toxic organic substances, to 100 %.
At a temperature over 375 ^oC and a pressure exceeding 220 bars, water passes into a fourth phase

– the supercritical phase – with properties between those of liquid and vapour. In this phase, and in the presence of oxygen, all organic material rapidly oxidizes. In less than a minute it decomposes into carbon dioxide and water and ammonium nitrogen transforms into nitrogen gas under heat generation. The inorganic material changes into its highest oxidation form giving an inert inorganic solid fraction containing mainly Fe₂O₃, SiO₂, P₂O₅ and Al₂O₃ plus a liquid phase with a very low concentration of impurities.

The inorganic phase coming out from the process is very simple to concentrate to more than 50 % DS-content. This phase, free from organic components, is an excellent raw material source for the recovery of for example coagulants and phosphorous.
A pilot plant for supercritical water oxidation in Karlskoga, Sweden, with a capacity of 250 l sludge/hour, have been used to conduct tests on sludge from Stockholm Sewage Works in order to decompose sewage sludge from the organics.
Tests have been conducted in order to investigate the possibilities of recovering phosphorous and iron coagulants. The tests showed that about 90 % of the phosphorous could be leached out of the sludge with caustic soda and subsequently separated into liquid form as sodium phosphate. From the liquid stream with addition of lime, calcium phosphate can be precipitated allowing the caustic soda to be reused for leaching of new phosphate. The precipitated calcium phosphate is very pure and free from heavy metals or iron originating from the coagulant.
From the solid residue it is thus possible to dissolve the iron oxide in acid and reuse it as a coagulant. As a result from the process, the organic content is transformed into energy. Phosphorous and iron can be recovered and all that remains are sand and gravel easy to dispose of.
In the paper, energy and material balances for the supercritical water oxidation process are presented together with a cost calculation for a full-scale operation.
57.

ABSTRACT
Drying and incineration of wastewater sludge – experiences and perspectives based on the development in Denmark
Niels Simonsen, Krüger A/S
During the last decade there has been an interesting development in Denmark with sludge drying and incineration plants. The main reason for this development has been increased demands in relation to the usage of wastewater sludge in the agriculture. These tendencies are expected to appear in the entire EU in the coming years.
The drying- and incineration development can be characterised by:

• 
  New concepts optimised for small and middlesize plants
  
• 
  Concepts with focus on energy efficiency
  
• 
  Environment friendly concepts
  

In general, commercial drying- and incineration plants have been available on the market for many years. However, these types of plants are not suitable for the many small WWTP’s that are searching for a solution for their own sludge problems. Therefore new simplified concepts have been developped.

An important approach when designing a plant is to regard the drying/incineration process as an integrated part of the total WWTP. Therefore the total sludge treatment must be optimised. This optimisation includes, among other things, digestion, utilisation of biogas, dewatering and heat recovery. Examples of how these analyses can be performed will be illustrated.
The most interesting drying/incineration concepts will be described and evaluated in the paper. The different types can be categorised in the following groups:

• 
  Drying plants (full drying)
  
• 
  Predrying integrated with incineration
  
• 
  Full drying integrated with incineration
  
• 
  Incineration without drying.
  

The types of equipment that will be considered are:

• 
  Dryers: Disc dryers, Belt dryers, Fluidised bed dryers
  
• 
  Incinerators: Grate furnaces, Fluidised bed furnaces
  

In the paper special focus will be on the experience obtained with the different concepts and the derived perspectives with regard to:

• 
  Flexibility: The degree of integration of the drying- and the incineration process.
  
• 
  Energy aspects: Primary energy consumption, possible energy sources, biogas utilisation and heat recovery.
  
• 
  Environment: Odour considerations, evaluation of operation and emissions compared to the EU-directive on incineration of waste.
  
• 
  Residues: Utilisation of dried sludge and slag/fly ash.
  

58.

A new process for cost efficient sludge minimisation and conditioning
Michael Wamsler
In light of the changed attitude of the Swedish Environment Protection Agency towards recovery of phosphorus from municipal waste water treatment sludge, Kemira Kemwater is developing a new technology for sludge minimisation, considerably cheaper and simpler than processes designed to recover phosphorus. The new process will be entirely cost driven, i.e. it will target only minimisation of sludge and creation of a solid fibre fraction, suitable for e.g. composting or firing in waste incineration plants. This means that the firing will not require any special arrangements and will not reduce the effectiveness and capacity of the incinerator.

To achieve this, an acidic hydrolysis is combined with a new application of an existing, well proven dewatering process with good results in terms of reliability and operation costs. The features of the process are:

• 
  Sludge minimisation
  A reduction by approximately half the weight, compared to a normal digested sludge at 25 % dry solids, reduces costs for transportation as well as fees for incineration and land filling.
  
  
  
• 
  Fuel fraction with good firing properties.
  The dry solids of a normal, chemically precipitated and digested sludge typically contains approximately 50 % ash. The new process aims to create a fibre fraction with a high, 40-50 %, dry solids and reduced ash content. This makes it ideal for handling and firing, and also results in an attractive heating value.
  
  
  
• 
  Small inorganic fraction
  A, small, inorganic fraction is created which can be deposited. It will, however, be low in heavy metals and organic substances, and therefore should be suitable for farmland application. From this fraction nutrients and other substances may still be recovered to make a commercial grade product in the future, if this is desired.
  
  
  
• 
  Clean reject
  The end product will be the reject from the process, which is returned to the waste water treatment plant. The reject will be low in contaminants and nutrients and therefore in most cases will not require any additional treatment before being returned to the waste water treatment plant.
  
  
  
• 
  Low pressure and temperature
  The temperatures in the new process will be low, removing the need for pressurised vessels and exotic materials.
  
  
  
• 
  Overall cost reduction
  All in all, the new process should be motivated by its financial benefits alone, by reducing costs and simplifying the life of the sludge manager by providing better possibilities for a safe and reliable recycling of the products.
  

59.

Influence of mechanical stressing of sludge on sludge growth
Georg W. Strünkmann¹, C. Lajapathi Rai^1,2, Johannes A. Müller¹, Jörg Schwedes^1
1 Institute of Mechanical Process Engineering, Post Box 3329, Technical University of Braunschweig, D 38023 Braunschweig, Germany, Tel. [phone], Fax [phone], E-mail: [e-mail]

² Central Leather Research Institute, Adyar, Chennai, India
Objectives: By mechanical stressing either the sludge flock can be destroyed or the cell walls of the microorganisms can be disrupted, depending on the energy input and the kind of forces. Both effects do have an influence on growth and degradation of sludge, in particular on:

• 
  Degradation of organic matter: The degradation of aerobic bacteria in an aerobic environment is a slow autolytic process. By disrupting the cell walls of the bacteria the intracellular components are released. This easily degradable substrate leads to an enhanced and accelerated degradation of sludge mass.
  
• 
  Growth-rate: Mechanical forces overcome the adhesive forces of extracellular-polymeric-substancies (EPS) leading to a reduced flock size and changes in flock-structure of the sludge. Substrate transport processes outside and inside the flock are influenced as well as the composition of the biocoenosis. The conditions for grazing organisms will be improved because of the higher number of non-aggregated organisms.
  

Methods: The mechanical stressing can be produced using different kinds of forces like shear- or pressure. For technical application various types of machines are used, which will be compared in the project like:

• 
  Ultrasonic homogenisers
  
• 
  High pressure homogenisers
  
• 
  Stirred media mills
  
• 
  Shear-gap homogenisers
  

Depending on the machine and operational parameters various types of stress are imposed on the sludge and the stress intensity can be regulated from flock-destruction to cell-disruption. The mechanical stress brought into the sludge is characterised by the specific energy input (E_spec), by the degrees of inactivation (DD_O) based on the oxygen uptake rates of the treated and untreated sludge and by the degree of COD release (DD_COD) effected by the stress.

The effect of mechanical stressing on the growth rate of sludge is tested with two batch sequenced respirometers. Within these systems the heterotrophic conversion yield (Y_H) of a known amount of degradable substrate into biomass and carbon dioxide can be measured. Lowering the conversion yield of the sludge by mechanical stressing means that less of the substrate added is used for building up new biomass and more is needed for non growth reactions and is converted into carbon dioxide.

Excess sludge production, biological, biochemical and physical properties of the sludge and effluent quality of the clarification process under stressing conditions are tested in lab-scale treatment reactors (20 - 30 L each) using activated sludge of municipal wastewater treatment plants. Loop-type reactors and a membrane reactor are operated in parallel with one Loop type set-up without stressing.
Results: Experiments done with the sequenced batch respirometers show that lowering the conversion yield of sludge is possible using mechanical stressing of sludge. Table 1 shows four results from measurements done with the sequenced batch respirometer. Two measurements were carried out after stressing the sludge with a shear-gap homogenizer and two tests after stressing with an ultrasonic homogeniser, with different specific energy inputs at each case. With the shear-gap homogeniser in both tests the conversion yield Y_H was lowered due to the mechanical stressing of the sludge. The higher specific energy input leads to an higher reduction of the conversion yield. In contrast to that with the ultrasonic homogeniser only at low specific energy input a reduction of the conversion yield could be achieved. A comparable high energy input than within the second test with the shear-gap homogeniser lead to an increased conversion yield if sludge was stressed with the ultrasonic homogeniser. These results show, that effecting sludge growth not only depends on the specific energy input but also on the machine used for stressing. This is obvious because different stressing systems do cause different degrees of inactivation and COD-release (DD_COD) when stressing sludge with comparable specific energy inputs as to be seen for the results of COD-release shown in table 1. Due to this not only the energy input but also the degrees of inactivation and COD-release have to be taken into account if sludge growth shall be reduced by mechanical stressing of sludge.
Table 1: Reduction of conversion yield by mechanical stressing of sludge

	Shear-gap homogeniser		Ultrasonic homogeniser
Espec [kJ/kg]	40,420	68,970	8,130	76,330
DDCOD [%]	2.7	3.22	7.22	23.37
YH,untreated	0,68	0,56	0,76	0,6
YH,treated	0,59	0,44	0,22	0,69
Change of YH [%]	-13.2	-21.,4	-71.1	+15

60.

SAPHYR^: a chemical stabilisation process of Vivendi Water Systems

Paulo Fernandes, Gilles Baratto, Lucie Patria. Anjou Recherche

Didier Crétenot, Jean-Pierre Levasseur. Vivendi Water Systems