World Student Community for Sustainable Development

by Ashraf Syed Ali, Mir Nadeem Ali

Supercritical Water Oxidation (SCWO) is an innovative, modern and effective destruction process for treatment of organic-rich sewage sludge. In the SCWO sludge treatment system, dilute aqueous organic waste is combined with an oxidant above the critical point of water, 22.1 MPa and 374.2°C. For most organic wastes, these conditions are sufficient to achieve more than 99.99% destruction and removal efficiencies and better in 30-60 seconds of reaction time. There are no harmful gaseous emissions to the environment (e.g., NOx and SOx) from the SCWO process. Trace amounts of nitrous oxide (N2O) are produced; however, these can be thermally or catalytically destroyed. Inorganic compounds leaving the SCWO process are oxidized to their highest valencies, which provides an opportunity for their recovery. Aqua Reci is a process in which SCWO is used to decompose the organic material, followed by a chemical process to recover valuable components from the inorganic residual ash, such as phosphates and precipitation chemicals. The aim of this paper is to analyze the technology of SCWO for handling of sewage sludge from Ryaverket Wastewater Treatment Plant (WWTP) in Gothenburg, Sweden.

Three scenarios were considered in this study: the current scenario, where a digester is used to produce biogas; SCWO of undigested sludge; and digester followed by SCWO. After performing a pinch analysis for the existing SCWO design patented by Chematur AB, it was found that pinch rules were violated and the process can be self-sufficient in terms of heat. After process heat integration, excess heat can be used for electricity and district heat production. A comparison was made of global carbon dioxide emissions for electricity generation, as well as district heating for the three scenarios. These values were compared to the carbon dioxide emissions if the same amount of electricity was produced by Natural Gas Combined Cycle at the grid station. It was found that the amount of carbon dioxide emission reductions for the three scenarios were 7,800 tons/year for the current scenario; 14,000 tons/year for the digester followed by SCWO; and 19,000 tons/year for SCWO of undigested sludge.

At the Ryaverket WWTP, phosphorus, iron, and silicate were chosen to be recovered due to economic considerations. For the SCWO scenarios, 310, 950, and 910 kg/day of phosphorus, iron, and silicate can be extracted, respectively. The large amount of carbon dioxide gas produced in the SCWO reactor as the final oxidation product can also be recovered. With 90% recovery of carbon dioxide gas, the amounts are 76 and 26 tons/day for SCWO of undigested and digested sludge, respectively.

Problem background

Sewage sludge is a by-product of municipal waste water treatment. Handling of sewage sludge has become an increasing challenge all over the world. The major problems of wastewater sludge handling are associated with the sludge volume reduction and upcoming constraints on the discharge of organic compounds and heavy metals into the environment. Sludge and sludge components may be deposited or used for different applications. Today, the following methods for handling sewage sludge are commonly in use:

* Deposition on land (e.g., municipal solid landfill site);
* Land application (e.g., soil conditioner, fertilizer);
* Landscaping and use as a construction material; and
* Incineration.

European Union (EU) regulations have been developed to ensure that public health and the environment are protected while handling the sewage sludge. Contaminated sludge or poor disposal practices pose a threat to public health and the environment and are subjected to enforcement practices. Thus, ocean disposal of sludge is forbidden today and sludge deposit in landfills will be phased out stepwise from the year 2005 in EU countries, including Sweden.

Until recently, most sewage sludge in Sweden was recycled for agricultural use and the remainder was generally discharged to landfills. The Farmer’s Union and the food industry, however, have recommended that farmers not use sludge in agriculture due to the possible presence of hazardous components. Presently, the methods available for complete destruction of organics in sludge are incineration and supercritical water oxidation (SCWO). Incineration is considered to be an effective method for sewage sludge volume reduction. Being economically comparable to the SCWO method, incineration has many disadvantages like high energy demand due to dewatering requirement; the need for flue gas cleaning to prevent gaseous emissions to the atmosphere; and, finally, the need to treat the ash produced by incineration as a waste. Thus, incineration does not offer a sustainable solution for sewage sludge handling, as it is environmentally and economically inefficient and faces social constraints.

SCWO is an innovative, modern, and effective destruction process for treatment of organic-rich sewage sludge. A major advantage of the SCWO system is its capability of almost completely destroying organic materials (>99.99%), including their toxic components. The time required for complete reaction is short, from a few seconds to a few minutes, which means high waste throughput for reactors. The recovery of valuable inorganic compounds (e.g., phosphorus, precipitation chemical, heavy metals) from the effluent can reduce the cost of the process as well as make it more sustainable.

This study is focused on the investigation of SCWO plant implementation at the Ryaverket Wastewater Treatment Plant (WWTP) in Gothenburg, Sweden. The sludge to be treated by SCWO is on-site generated, digested, and undigested sludge, as well as hazardous waste streams collected within the Gothenburg area. The issues discussed in this report include process selection and environmental and other sustainability issues.

Choice of the SCWO technology

In the engineering practice of process design, the properties of sewage sludge must be considered in order to achieve an efficient, reliable, and economic method of treatment. After studying the composition of the sewage sludge that is generated at the Ryaverket WWTP, as well as available SCWO technologies, we have concluded that the components such as phosphorus, oxygen, and carbon dioxide are worth recovering from the SCWO outflow to make the overall process more cost-effective. The Aqua Reci process was taken as a basis for our design. Aqua Reci is the process where SCWO is used to decompose the organic material, followed by the recovery of outlet gases, as well as a chemical process to recover the valuable components from the inorganic residual ash such as phosphates and coagulants. In the SCWO sludge treatment system, dilute aqueous organic waste is combined with an oxidizer above the critical point of water, 22.1 MPa and 374.2°C. For most organic wastes, these conditions provide more than 99.99% destruction and removal efficiencies and better in 30-60 seconds of reaction time. This is due to the fact that the supercritical water (SCW) has properties which are between those of a gas and a liquid. The density of SCW is similar to the density of liquid water and is high enough for reasonable throughputs in a process. On the other hand, the viscosity and diffusivity in the supercritical region are more like that of a gas. SCW also has a high solvating power due to its low dielectric constant.

The Aqua Reci process design is shown in the following figure (Figure 1 [1]):

Untreated sludge enters the stir tank, which produces a more homogeneous, viscous sludge. The outlet stream from the tank is passed through a macerator to ensure a constant supply of sludge, without big particles, to the high-pressure pump. The high-pressure pump is used to raise the feed pressure to about 25 MPa. An economizer is used to preheat the pressurized feed with the reactor effluent. A heater is then used to heat the feed above 330°C before entering the reactor. Next, the feed enters the reactor and oxygen is injected to start the oxidation reaction. The reactor outlet, which is kept at a temperature of about 600°C, is used for preheating the feed. A steam boiler is used to recover the remaining reaction heat for the generation of steam of specified quality. Further on, the effluent temperature is decreased with a cooler and pressure is released using pressure reduction coils until atmospheric pressure is achieved. Gases (carbon dioxide and oxygen) and water with inorganics are separated in the gas/liquid separator. Further separation of the gases for carbon dioxide and oxygen recovery is achieved by distillation. The main reactions Aqua Reci uses to recovery phosphorus are acidic and alkaline. Preference at Ryaverket WWTP was given to the acidic leaching method due to the high calcium content in the sewage sludge processed there. Thus, after the sludge residue is thickened, sulfuric acid is added to the leaching phosphorus.

Recovery of inorganics

As described above, Aqua Reci recovers phosphorus mainly through acidic and alkaline reactions. The choice of the leaching method is strongly dependent on the calcium content of the sludge. Alkaline leaching cannot dissolve the calcium phosphate present in the inorganic residue. Thus, due to the high calcium content in the sewage sludge from Ryaverket WWTP acidic leaching method was chosen to recover such inorganic compounds as phosphorus, iron, and silicate from the inorganic ash. With the use of sulfuric acid, it is possible to leach both phosphorus and metals to virtually 100%. [2] Sulfuric acid is usually chosen for extraction because it is inexpensive.

Through leaching with sulphuric acid, the solutionâ€^TMs pH will drop to about 1.3 when it contains all the phosphorus and metals. One way to separate the components in the liquor is a two-step neutralization process using magnesium oxide. In the first step, the pH is adjusted to around 2, where the trivalent iron phosphate (FePO4) is virtually insoluble while all other metals will remain in solution. This means that a pure ferric phosphate can be separated. [3]

In the second neutralization step, at a pH of 7-8, all heavy metals can be precipitated. After separation of the heavy metals, the remaining water leaving the process is eliminated from all phosphates and metals. [4] Such a process can be designed according to Figure 2.

After the acid leaching process, the insoluble matter, mostly silicate, can be separated from the solution through filtration or centrifugation.

Further on, iron phosphate is separated into iron and the corresponding phosphate salt. An attractive option is to continue to dissolve the ferric phosphate in hydrochloric acid. Ferric chloride can then be separated from the phosphoric acid with the help of liquid extraction. The liquid extraction process will separate the iron from the phosphorus to practically 100% and the phosphoric acid extracted will be pure. [5]

Sustainability discussion

The SCWO process is discussed below according to three interdependent sustainability “pillars†: environment, economy, and society. From an environmental point of view, the SCWO method is indeed remarkable, as it can transform organic sludge into non-hazardous products. The most prominent feature of this technology is the complete destruction (>99.99% COD removal) of all organic material, including its toxic components. Thus, carbon, hydrogen, and nitrogen atoms are completely mineralized to carbon dioxide, water, and molecular nitrogen (N2), and halogenated and sulphonated organics are converted into the corresponding acids. Inorganic compounds, on the other hand, stay inert, which provides an opportunity for their recovery after the oxidation reaction. In this case study, the recovery of the finite resource phosphorus is a good alternative by use of sludge in agriculture. As a whole, the extraction of valuable inorganic compounds from the process is a good attempt at the sustainable handling of waste streams.

Another strong advantage of the SCWO process is that there are no harmful gaseous emissions to the environment containing NOx and SOx. Trace amounts of nitrous oxide (N2O) are produced, but they can be thermally destroyed and are not considered to be an acidifying substance. The content of volatile organic compounds (VOCs) is negligibly small as compared, for example, with that produced by incineration. The economic value of the process is, therefore, attractive, as there is no need for the additional treatment equipment for gaseous outlet cleaning. The recovery of carbon dioxide and oxygen from the outlet gaseous stream of the process is possible, which makes the process friendlier in terms of resource use and economics. The excess energy produced by the exothermic SCWO reaction at the Ryaverket WWTP can be used in the process and/or used for district heating in the Gothenburg area.

Safety and risk assessment is another important issue in a sustainable society. The SCWO process claims to be enclosed and controlled: the system is not in contact with the environment and can be quickly shut down. Human risks are of a low probability because of the plant automation system and its installation in the industrial zone.

Conclusion

SCWO technology for sewage sludge treatment at Ryaverket WWTP is an attractive option. It has been estimated that the capacity of the existing plant is sufficient to adopt this new technology. Moreover, the installation of the SCWO plant on the Ryaverket WWTP area will bring environmental, economic, and societal benefits such as:

* Significant reduction of the sewage sludge volume;
* Complete destruction of organic compounds in the sewage sludge;
* The possibility of the process being self-sufficient in terms of heat demand;
* Significant reductions of carbon dioxide emissions through the use of extra heat, which is available within the process, to produce “green†electricity and district heating; and
* The possibility of recovering (and selling) substantial amounts of carbon dioxide and inorganics such as phosphorus, iron, and silicate.

Based on the results of this study, it can be concluded that the SCWO of undigested sludge is a better environmental option in terms of reductions of carbon dioxide emissions when compared with the other scenarios (the digester used to produce biogas). It is recommended that the SCWO technology be installed after the digester to treat the digested sludge at the Ryaverket WWTP.

- Mir Nadeem Ali and Ashraf Syed Ali, Master’s students in Environmentally Sustainable Process Technology, School of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden; originally from Pakistan and Bangladesh, respectively
Footnotes

[1] Chematur Engineering AB, “SCWO: Supercritical Water Oxidation,†http://www.chematur.se/sok/supercrit_scwo.htm

[2] K. Stendahl and S. Jäfverström, 2002, “Recycling of Sludge with the Aqua Reci Process,†Water Science & Technology, 48(1): 185—190

[3] Ibid.

[4] Ibid.

[5] Ibid.
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