Iwa international Specialist Conference

Abstract

Odour control and dewaterability are the key criteria during biosolids storage either for use on land or incineration. In the case of use on land, stabilisation/sanitisation are also part of the key criteria.
Vivendi Water Systems developed the SAPHYR^ process to answer those three requirements. The SAPHYR^ process principle is based on an acidification of biosolids associated to the addition of nitrite. The main results are a noticeable odour control lasting other periods of 6 to 12 months, an improved dewaterability (2 to 4 points of dryness) and depending on chemical dosages a stabilisation or a sanitisation of biosolids.

Another characteristic is that biosolids conditioned with the Saphyr process can be used both on land or for incineration.

After several demonstrations on more than 5 different plants throughout France, the process was installed on a 50 000 population equivalent wastewater treatment plant in 2002 and will be in operation by November 2002.

A close monitoring of the process operation, the biosolids quality and its storage and spreading on land is planned from November 2002 to spring 2003. A comparison with lime addition will take place on the same plant.

The present paper will produce a presentation of the SAPHYR^ process, its operation on a 50 000 pe WWTP and its different applications for biosolids storage.

61.

Optimizing Waste Heat Recovery for Class A biosolids Production from a Combined Cycle Power Plant

Fred Soroushian, CH2MHILL

Paper Abstract for IWA International Conference ”Biosolids 2003 – Wastewater Sludge as a Resource”

Key words: Sludge, Heat Drying, Waste Heat Recovery, HRSG, Class A Biosolids

Contact Information: Fred Soroushian, CH2M HILL

3 Hutton Center Drive, Suite 200

Santa Ana, CA 92707 USA

E-mail: [e-mail], Fax Number: 714-424-2232

The City or Corona serves a rapidly growing area of Southern California. The City operates three wastewater Treatment plants (WWTP) that produce reclaimed water meeting California Title 22 reuse requirements. The sludge from the three WWTPs are transported to a central sludge treatment facility located at WWTP No. 1. The sludge treatment facility consists of sludge receiving, thickening, anaerobic digestion, and dewatering.

In year 2000, the City was faced with two crisis. First the California power shortage and escalating cost of power, impacting the industry, businesses, and the City’s prosperity. Second, bans on Class B biosolids land application and shut down of a privatized composting facility where bulk of the City’s biosolids were processed or reused. To cost effectively respond to these crisis, the City decided to start generating and supplying power to it’s constituents by constructing a 25 MW power plant and by converting the biosolids operation to produce Class A biosolids. The feasibility study proved that locating the power plant at WWTP No. 1 produced significant synergies. The reclaimed water from the WWTP could be used for power plant cooling, the waste heat from the power plant could be recovered and used in Class A biosolids processes, the digester gas could be used for supplementing the power plant’s fuel needs, and the combined facilities operation was more efficient than physically separate facilities. This paper presents the results of this analysis as well as the construction and operational aspects of the project.

The Class A biosolids that are able to utilize waste heat from the cogeneration power plant include thermal hydrolysis(Cambi Process), heat drying , thermophilic digestion, and heat pasteurization. Based on the evaluation of these alternatives, thermal hydrolysis (using steam at approximately 150 psig and 300 degrees F) and heat drying were selected for further evaluation. Heat drying of sludge to produce Class A biosolids can be achieved using direct or indirect heat dryers. Direct rotary drum heat dryers proved to be more efficient for this application due to their higher efficiency and ability to produce more evenly dry pellets and was selected. Direct heat dryers blow hot air (between 1100 and 350 degrees F) directly on the sludge to remove moisture.

The 25 MW power plant is a combined cycle plant consisting of a gas combustion turbine generator (CTG), a heat recovery steam generator (HRSG) with Selective Catalytic Reduction (SCR), a steam turbine generator (STG) and all the ancillary equipment. Waste heat can be recovered from the system either downstream of the STG or at the HRSG unit. The waste heat available downstream of the STG is in the form of spent steam suitable for thermal hydrolysis process (approximately 150 psig and 300 degrees F). This process consumes the steam, which must be replaced to maintain the power production. Steam hydrolysis improves digester gas production and digested sludge dewaterability. It also produces a Class A biosolids. The City needed to further dry the dewatered sludge cake to reduce hauling cost and produce a product that could be marketed as fertilizer. The steam withdrawal would also reduce power production by about 0.3 MW. Therefore, the steam hydrolysis process was not considered any further.

The waste heat available at the HRSG is in the form of waste hot gas after steam production for the STG. The waste hot gas, which contain between 14 and 15 percent oxygen, flows through the SCR to reduce pollutants before disposal through the vent stack. The temperature of the waste hot gas depends on where it is withdrawn from the HRSG unit. The waste hot gas cools off as it flows from the steam generation system through the SCR to the vent stack (from approximately 900 to 250 degrees F). To optimize energy production in the combined cycle cogeneration power plant, the waste hot gas was withdrawn downstream of the SCR at a temperature of approximately 500 degrees F. Withdrawal of waste hot gas at this stage reduced the power production by about 0.1 MW/day. Since the waste hot gas downstream of the SCR has been treated for pollutants, it was easy to dispose of it after using it in the heat dryer. Due to the high oxygen content in the waste hot gas and large gas volume, it was more economical to use a heat exchanger between the cogeneration power plant and the heat dryer to extract the energy from the waste hot gas and heat the air required in the heat dryer. The heated air was combusted, using some of the digester gas, to increase its temperature and reduce its oxygen content to less than 5 percent and, then, used for drying the sludge.

The selected for this project consist of installing a direct rotary drum heat dryer and a nominal 10 MW cogeneration power plant. The heat dryer will require approximately 11.8 million Btu/hr of thermal input to evaporate 8200 lbs of water per hour from the estimated 18.8 tons of dry solids per day treated at WWTP No. 1. To reduce the construction costs and expedite the facilities construction, City decided to use turn-key construction approach for heat drying system, to pre-select and pre-purchase the major components of the power plant, and to retain a general contractor for installation of the power plant components and piping. This paper also discusses the methodology, cost savings, construction schedule acceleration and other efficiencies achieved through these methods of procurement and construction.
62.

ABSTRACT
Chemical Conditioning of Sludges
John T. Novak and Chul Park

Department of Civil & Environmental Engineering

Virginia Polytechnic Institute & State University

Blacksburg, VA 24061 USA

Introduction. Recent studies at Virginia Tech have resulted in a proposed floc model in which the binding of biopolymer within a floc is controlled by cations. Two types of biopolymer binding are proposed to exist. Calcium and magnesium appear to be linked to lectin-like proteins. Lectins are proteins that are generated by bacteria for flocculation or binding to surfaces. These proteins are linked directly to polysaccharides by divalent cations. More recently, it has been found that iron and aluminum also have a major impact on floc structure. It has been proposed by Park, et al. (2002) that iron and aluminum serve as collectors of biopolymer generated by cell growth and lysis or contained in wastewater influent. Data presented in Figure 1 shows the relationship between iron and aluminum in floc and effluent biopolymer content for different activated sludge plants.

Figure 1. Effect of Floc Fe and Al on effluent Biopolymer

When biological sludges are digested, flocs are disrupted, leading to a high polymer conditioning demand. In this research, we have been conducting studies to better understand how the composition and structure of the floc affects the conditioning and dewatering behavior of digested sludges. A variety of sludges were digested anaerobically and aerobically and the dewatering characteristics and conditioning requirements measured. These data will help us understand why some digested sludges are much poorer than others and why some conditioning agents work better than others.
Results It was found that anaerobic digestion produced large quantities of soluble colloidal protein that interfered with dewatering. These colloidal proteins also determined the dose of conditioning chemical needed for dewatering, as is shown in Figure 2. The colloidal range was defined as passing a 1.5μm filter but retained on a 30k ultrafiltration membrane. For aerobically digested sludge, conditioning requirements were more dependent on the combination of colloidal proteins and polysaccharides. The reason for the difference in colloidal material had to do with the floc structure. Under aerobic conditions, biopolymer associated with calcium and magnesium were released while for anaerobic digestion, iron associated proteins were released into solution. Therefore, the amount of iron determined conditioning requirements for anaerobically digested sludges while calcium and magnesium dictated aerobically digested sludge conditioning requirements.

Figure 2 Relationship Between Colloidal Protein and Conditioning Dose

The effect of colloidal biopolymer in digested sludges and specific resistance is shown in Figure 3. The data in Figure 3 is a combination of waste activated, anaerobically digested and aerobically digested from 7 plants.

Figure 3. Effect of Colloidal Biopolymer on SRF.

Additional data will be provided showing the relationship between floc structure, the release of colloidal biopolymer into solution, and the effect this has on conditioning by both polymers and metal ion conditioners. The amount of biopolymer released and the chemical content of biopolymer will be shown to depend on the cation content of the floc and whether aerobic or anaerobic digestion is used. These data can be used to predict conditioning requirements for waste activated sludge and digested sludges from activated sludge plants.

63.

EXPERIENCE AND LESSONS LEARNED FROM SEWAGE SLUDGE PYROLYSIS IN AUSTRALIA

Trevor Bridle

Environmental Solutions International Ltd, 21 Teddington Road, Burswood 6100, Perth, Western Australia.

Dr Stefan Skrypski Mantele

Environmental Solutions International Ltd, Schenkenzell 77773, Germany.

ABSTRACT: Management of sewage sludge via “publicly acceptable” methods is becoming increasingly difficult primarily due to health and environmental concerns with respect to reuse of the product in agriculture. Consequently thermal processes are gaining popularity with significantly increased interest being shown in pyrolysis and gasification processes, due to their “non-incineration” status.
Over the past 15 years Environmental Solutions International Ltd (ESI) has developed and commercialised a pyrolysis-based process, called ENERSLUDGE^TM for the conversion of dried sewage sludge to liquid, solid and gaseous fuels. The process involves the indirect heating of dried sludge in the complete absence of oxygen to about 450 ⁰C at a pressure of between 2 and 5 kPag. This generates a “crude” pyrolysis gas and char which are re-contacted in a second reactor, also operated at 450 ⁰C to facilitate catalysed vapour phase reactions to decarboxylate and deaminate the lipids and proteins which were “distilled” from the sludge. All sewage sludges have the necessary catalysts (alumino silicates and heavy metals) inherently present. The process recovers all of the sludge energy in the four fuels produced, namely oil, char, non-condensed gas (NCG) and reaction water (RW).
The world’s first commercial ENERSLUDGE^TM plant located at the Subiaco Wastewater Treatment Plant (WWTP), was recently handed over to the client, the Water Corporation of Western Australia. The plant is designed to process all the raw

primary and secondary sludge produced at the WWTP, which currently ranges from 16

to 20 dry tonnes per day (tpd). The plant comprises sludge dewatering, sludge drying, conversion to oil, energy recovery and gas cleaning. The thermal energy needed to dry the sludge is provided by combusting the char, NCG and RW in a fluid bed Hot Gas Generator. The oil is used as an industrial fuel off-site in boilers, kilns and furnaces.
After commissioning, the integrated plant was operated for a 9 month period by the contractor before hand-over to the client. During this period intensive monitoring was conducted to verify the process, environmental and energy benefits offered by the technology. In addition “real-world” cost data was developed to confirm operating and maintenance costs for the plant. Data has shown that the plant delivers a nett thermal energy output of 7.7 GJ per dry tonne of sludge processed, after providing the energy for sludge drying. Environmental monitoring has clearly shown the unique ability of the process to control the contaminants in sludge, notably heavy metals and organochlorine compounds, resulting in very low atmospheric emissions from the plant. In addition the process produces an inert ash with most of the sludge heavy metals securely speciated as non leachable oxides and silicates. Finally, cost data from the plant has shown the importance of maximising revenue from the oil product.
The paper will provide technical details on the plant and summarise the operational experience gained and lessons learned from this “world first” facility.