By William T. Wofford
Summary: For cities and companies in the business of maintaining wastewater treatment facilities, sludge disposal is often a very tedious and costly proposition. Finding the right technology to do the job as well as paying heed to the bottom line are often at odds with one another. One process described here takes an innovative approach to the sludge issue and offers an alternative way of tackling this growing concern.
Federal and state environmental agencies are taking a close look at the health and safety issues of sludge disposal. Historically, industry has leaned toward the use of incineration, boiler or industrial furnaces (BIFs) or thermal decomposition to destroy sludge. A boiler burns fuel to provide steam while an industrial furnace heats process air or fluids directly or indirectly, usually by direct firing. Sewage sludge has also been air dried and sent to land disposal after the sludge volume is reduced by the use of aeration basins, sequential batch reactors, dissolved air flotation, filtration and thickening. These are essentially solid/liquid separation processes that create sludge—and treat water for reuse or discharge as wastewater that meets federal guidelines. With more water treatment dealers getting into commercial/light industrial and other small system applications, the topic is one with which they should become familiar.
Ultimately, process selection for both wastewater treatment and sludge disposal boils down to economics. With fuel costs of trucking sludge from the sewage treatment plant to a disposal site increasing from $1.80 per mile to over $2.80 per mile (per truck) over the past two years (regardless of the wetness or dryness of the sludge being hauled), transportation considerations are much more critical than in the past. Also, any thermal process that spews out air pollutants such as particulates, heavy metals, unburned hydrocarbons (UHCs) or nitrogen oxides (NOX) carries an environmental stigma that’s undesirable.
Sludge can be sold to farmers or others. In some cases, a city actually pays a farmer to distribute municipal sludge onto the land. All organic industrial sludges burn; the industrial sludges that don’t burn are inorganic sludges such as those from pigment, mineral or caustic operations.
As a result, final disposal of sludge is one of the more pressing problems for companies and cities operating wastewater treatment facilities. Present regulations for municipal sludge have favored beneficial use, defined as having a positive impact on the environment and/or economic benefits in lieu of costly disposal. However, potential health risks and concern for living matter that may block flow have led engineers to consider new technologies for conversion or disposal. One alternative method is a process that’s a spin-off of high pressure, high temperature oxidation that converts all organic matter to carbon dioxide (CO2) and treated water (see Figure 1).
Inorganic materials, such as heavy metals, pass through the system and are converted to non-leachable, innocuous (or harmless) inert materials. The first unit is currently being installed in a Harlingen, Texas, sewage treatment plant. Test results on the sludge indicate conversion rates of over 99.99 percent, with no NOX emission problem.
Pretreatment, such as grit removal, occurs prior to sludge entering the process, which has six operational sections or unit operations. These are:
- Feed preheating
- Conversion reaction
- Cooling and energy recovery
The key equipment components are pumps, tanks, heat exchangers, mixing devices, reactor, pressure reduction hardware, a cooling system, and process control sensors and instrumentation. Integral to the operation, but not part of the actual process, is an oxygen supply and delivery system.
The British thermal unit (Btu) heating value of the organic sludge creates a self-sustaining oxidative condition after an initial startup assist from a natural gas furnace. At Harlingen, the sludge is approximately 6 to 8 percent solids by weight and, on a dry basis, this sludge has a heating value of approximately 950 Btu per pound. (The process can actually handle up to 24 percent solids by weight.)
The process flowsheet can be modified to include additional heat exchangers for energy recovery in the preheating section, and effluent cooling can also include additional energy recovery operations. Depressurization is commonly accomplished by standard, letdown and pressure control valves along with the assistance of capillary pressure reduction, which takes place in equipment that passes the stream through a series of tubes distributing pressure over a boundary area to achieve a lower pressure from a higher pressure.
In the process, homogenized sludge is fed to a high-pressure pump where it reaches 25.9 MPa (megapascals)—3,750 pounds per square inch (psi). After passing through two heat exchangers, oxygen (O2) gas is added by utilizing a proprietary feed technology before entering the reactor. Oxidation occurs as the sludge stream travels through a gas-fired heater and the reactor (see Figure 1). The reactor provides sufficient retention time at supercritical conditions to fully oxidize the organic material to CO2 and water in the form of high-pressure steam. Besides water, CO2 and excess O2, the process effluent—depending upon operating conditions and the quantitative analysis of the feed—may contain small amounts of ammonia and nitrogen that shouldn’t affect disposal permits. There are no high temperature effluents or exhaust gases that are environmental problems.
By-products and streams
At Harlingen, there’s a nearby textile plant with needs that are quite synergistic with the output of the process. To understand this symbiosis, one should understand the flow rates and material balances associated with the sludge conversion plant and the associated wastewater treatment units that feed the process.
First, the process is tied to and handles all of the sludge from three of Harlingen’s wastewater plants that have a total treatment capacity of about 14 million gallons per day (mgd). The sludge inflow from these facilities requires that the total sludge treatment capacity be approximately 26 gallons per minute (gpm), which represents only the sludge flow. The sludge feed consists of undigested sludge (biosolids) from the city’s primary and secondary clarifiers, as well as septage sludge and grease trap waste (an additional source of income). As noted, the feed will enter the unit at between 6 to 8 percent total solids. The reactor operates at a temperature of 950-1,100°F and a pressure of 3,400 pounds per square inch absolute (psia) and is totally self-sustaining after an initial (gas fired) energy input that starts sludge oxidation. Residence time is only 30 to 45 seconds at hydrothermal conditions. This can be as high as 75 seconds for some industrial sludge. A highly organic industrial sludge will require more treatment/oxidation time due to the higher organic (carbon) load.
Economics and payback
The technology is a new process, but the technology isn’t without operational history. U.S. Navy research of high pressure, high temperature, oxidation technology has proved successful on all types of shipboard wastes from solvents to paint. A key advantage in both industrial and municipal operations is that it can eliminate very expensive digestion and dewatering operations. The postscript to this elimination is that downstream wastewater processing equipment can be sized smaller allowing capital costs savings. Correspondingly, using this process to retrofit an existing wastewater treatment facility will allow a plant’s current equipment to handle more capacity without additional capital outlays. There’s a capital outlay to build a plant. Effectively, there are six bonus features.
At Harlingen, the overall economics get a boost from saleable by-products of steam energy and CO2. Both are used by the neighboring textile facility for energy use and wastewater treatment, respectively, with the CO2 providing a neutralizing agent for any caustic effluent. To put this in perspective, the capital cost of the facility was $8 million, and operating cost is about $180 per dry ton (pdt), versus about $275 pdt for the cost of sludge disposal by land application. Harlingen’s waste heat recovery credit plus its carbon dioxide credit amounts to $120 pdt. This reduces net operating costs to about $60 pdt. A breakdown of operation and maintenance (O&M) costs shows seven major O&M components (see Figure 2). For both industrial and municipal uses, one can make a comparison of anaerobic vs. aerobic vs. the process described.
While not a panacea, supercritical water oxidation offers industrial, chemical and municipal plants a non-polluting alternative to other thermal treatment techniques without the stigma of releasing nitrogen oxides, particulate or heavy metals. The treated water produced, when stripped of residual carbon dioxide, is ideal for makeup water for process or cooling applications, and the CO2 gas can be utilized to neutralize caustic effluents (often present in textile and petrochemical plants).
Overall economics will vary depending upon the sludge being processed and destroyed or otherwise disposed of, as well as whether the facility chooses to make use of by-product energy or electricity production credits from the steam generated. A big bonus is the savings in hauling costs to take sludge to disposal sites.
About the author
William T. Wofford, Ph.D., is director of research and development at HydroProcessing LLC, of Austin, Texas. He can be reached at (512) 339-9981, (512) 339-9827 (fax) or email: firstname.lastname@example.org.