Disposal of Municipal and Animal Biosolids Using Deep Injection

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Deep Biosolids Injection

Disposal of Municipal and Animal Biosolids Using Deep Injection

Maurice Dusseault

University of Waterloo

Waterloo Ontario Canada

Introduction: The human and animal biosolids treatment debate revolves around safe and environmentally- benign long-term disposal. The record for animal biosolid waste is seriously blemished: it appears that the disposal processes available and used throughout the world are inherently high risk and can therefore easily lead to groundwater and soil contamination. There is’s a new emerging technology that can reduce environmental risk and can also provide several additional benefits. Managers of animal waste streams in any region of animal husbandry concentration should be interested in learning about deep biosolids injection (DBI).

Background

Municipal or animal biosolids slurries can be intercepted before complete aerobic digestion and injected deep into porous, permeable strata using a cased well, a technique first developed for non-hazardous oilfield waste disposal. DBI forces the slurry into fractures that are continuously generated by the elevated injection pressures. A single well can process over 1,000 m3 a day and perhaps accept 500,000 m3 of slurry (i.e. about 100,000 m3 of solids) or more over a period of several years. The capacity for a single site using several injection wells should handle direct animal biosolids for 30,000 – 40,000 cattle (including urine and liquid phases), or provide municipal secondary sludge disposal for a city of about 500,000 population. Figure 1 shows a picture of a site in Alberta that was active for several years for solid oilfield waste disposal. The entire system can fit on a site less than a hectare, including holding tanks and other facilities.

For a municipal injection system, it will be necessary to have a pretreatment that strips off excess water, homogenizes the waste, and detoxifies it to render it safe for handling. Once this is done, the homogenized slurry is directly injected at a density of ~1.15 to 1.25 g/cm3.

The injected biosolid waste becomes compacted because the overburden weight forces excess water into the surrounding porous and permeable strata; then, it degrades slowly into gases and elemental carbon through anaerobic decomposition. Figure 2 shows the process at depth, including the location of the wells to inject waste and to harvest the generated methane.

Advantages

DBI has benefits that make it an interesting alternative to conventional surface treatment or field spreading of animal waste. Injected biosolids decompose anaerobically and generate methane. Perhaps 80% of the gases generated will be methane, the rest largely CO2. Thus, not only is CO2 emission from conventional biodegradation reduced, but also high quality and valuable dry fuel gas is generated for beneficial use. This is energy recycling at its most fundamental level, and it is ecologically sound and environmentally attractive.

Deep placement means that odor problems and land use issues such as large settling ponds, aerators and spreading of liquid manure on fields can be eliminated. This means that the risk of contamination of groundwater wells is virtually eliminated, and the possibility of accidents and hazards arising because of human error can be dramatically reduced.

Although municipal biosolids are rich in heavy metals and noxious trace chemicals, animal biosolids are generally less so, but both types of biosolids have a substantial content of viruses, prions, and perhaps even bacteriological pathogens such as E. Coli. At depth, high temperatures kill these agents or else they remain permanently isolated, filtered out through the processes of slow flow.

CO2 and other gases produced by deep biodegradation (except for methane) dissolve readily into natural formation waters because of their solubility and the high pressures. The dissolved CO2 is permanently sequestered in the aqueous phase.

The biosolids decompose until all the available hydrogen and oxygen are consumed. A solid carbon-rich residue containing about 30 to 35% of the original dry organic mass remains behind, permanently sequestered by the overburden weight. Generation of elemental carbon is analogous to the natural processes that generated coal beds millions of years ago, except that through immediate deep burial, the process is greatly accelerated.

Suitable geological conditions for DBI can be widely found. For example, major European cities sitting on suitable geology include London, Amsterdam, Brussels, Hamburg, Milano, Marseilles, Aachen, Bordeaux, and many other large cities. In North America, suitable conditions are found in the Los Angeles Basin, where DBI will first take place in 2004[1], as well as in cities such as Atlanta, Houston, San Francisco, Denver, Philadelphia, Dallas, Boston, and many others. DBI should be feasible even if there are only thin layers of suitable sediments (250-400 m).

Disadvantages

Injection technology has been used for over a decade in the oil industry for waste materials such as drill cuttings or produced sand; however, degradable sludge has yet to be disposed of by this means. This means that issues related to the systematic and continuous handling of a biological agent during DBI have yet to be addressed in practice.

DBI technology is not feasible for all sites. The geology and deep formation hydrodynamics must be carefully studied to ensure that injection can take place safely. An ideal site must have a suitable porous and permeable injection stratum, there should be thick shales or other low permeability zones above the injection bed, the injection stratum must be deep enough to be far below potable groundwater, and so on. This criterion disqualifies some areas such as the Canadian Shield and much of Scandinavia. However, it is also noted that such areas are in general not suitable for large animal husbandry industries because of the lack of fodder.

The DBI process requires high-pressure injection, so the pumping equipment must meet certain standards, as slow or low-pressure injection may not be feasible. Oilfield pumping technology must be used so that continuous fracturing takes place at depth. At least one dedicated DBI well is required (two or three wells are preferred for a single injection unit) and old wells in depleted oil and gas fields may be used if they are properly analyzed and meet certain criteria.

It will usually be necessary to drill a new well and complete the well with 140 mm or 175 mm steel casing cemented into place. A 61 mm to 88 mm 2diameter by 2-inch [CORRECT?] tubing with a bottom-hole pressure gauge and a packer must be installed for injection. Although the high pressures dissipate rapidly in the right formation, monitoring and proper operation of the system is needed to avoid problems and to satisfy regulatory needs.

Likely, a DBI facility would have to be capable of disposing of at least 100,000 m3/year of sludge slurry to be economical; small facilities are not likely to be suitable. Fortunately, this number is easily achieved, and much larger volumes of oilfield wastes have been disposed using this approach. For animal biosolids, direct injection of liquid manure and slurried wastes could take place, but perhaps an initial stage of slurry conditioning would be required before injection.

Finally, it should be noted that DBI uses petroleum industry technology, and there are many areas where DBI will be suitable (Hamburg, Denmark) but there is no petroleum industry infrastructure. This is not a serious handicap, but it can affect installation and operating costs.

Costs

Currently, animal biosolids are disposed by generating liquid slurry and land spreading. Recent developments include injecting the biosolids 75-150 mm below the surface of the soil to eliminate odors and surface spraying. Direct surface land disposal is a relatively low cost approach to waste management, but it is also a high-risk approach, and many accidents have happened in the past. Any rational assessment of true cost must take into account the environmental liability associated with these risks, and the additional risk to humans from a potential accident. Because it is impossible to monitor all or even a few spreading operations, it is difficult to manage these risks. There is thus much debate as to what are the true costs to society arising from current disposal methods.

For municipal biosolids, direct costs are far better known, and range from US$20.00 to US$120.00 per dry tonne of sludge. This vast difference in costs depends on two factors: whether full biodegradation and removal of excess phosphorous and nitrogen are required (tertiary treatment), and where and how the sludge is to be disposed. For example, to solidify municipal sludge after secondary treatment (biodegradation) using a vacuum screen, transport the sludge to a licensed landfill, and dispose of it by paying tipping fees engenders a cost of from US$30.00 to US$50.00/tonne in the United States. Because of the high heavy metals content and traces of compounds such as polychlorinated hydrocarbons, land spreading requires large areas and thin applications repeated only at long intervals. For example, the City of New York sends substantial amounts of its sludge to poor quality forest land in the state of Virginia, hundreds of kilometers distant, at considerable cost (excellent DBI sites exist within 50 km of New York, even though the City itself does not have suitable geological conditions). These high transportation distances result in total biosolids treatment costs of more than $100.00/t.

Costs for animal biosolids disposal are now rising substantially because of recent changes to the regulatory standards. It is not clear at this time what the costs for animal biosolids will be, nor is it entirely clear what the costs to a small farm operator will be for approved waste handling. There are economies of scale in handling wastes that make large-scale operations less costly than small farm operations, but the details of these costs under the new rules are not yet clear.

Large-scale animal waste disposal using DBI can probably be achieved for a unit slurry volume cost of perhaps $20 per cubic metre of liquid animal slurry provided. Municipal wastes could be co-disposed as well. The major unit costs for a DBI site include approximately $850,000 for two cased wells 400-800 m deep, perhaps $1,500,000 for the slurry mixing and injection unit, and perhaps another $1,500,000 for the surface facilities. If a sewage pretreatment facility is needed, this will cost an additional $5,000,000. This total is nonetheless a fraction of, for example, a complete municipal waste management facility, and the solids are disposed with far lower environmental risk levels.

If true disposal costs are considered -- including the costs of potential groundwater degradation, surface land use impairment, greenhouse gas generation, odor, and increased health risks -- DBI is fully competitive with the alternatives, and likely to be substantially cheaper than standard municipal sewage waste treatment approaches.

Mtethane generation and harvesting is also a valuable asset, but alone cannot pay for DBI costs. Methane is useful for electricity generation and heating, and it is considered to be the most benign of the hydrocarbon energy sources. Given current natural gas prices, it is believed that methane generation may defray up to 40% of the costs of a DBI system.

Acceptance?

The DBI concept is new. A clear understanding of deep injection of slurries is a recent development, and reliable well-designed equipment that can inject slurries continuously under high pressure has only existed for about a decade.

Municipal engineers, agronomists and agricultural engineers are conservative and study new technologies carefully before adopting them; after all, current methods, despite limitations, are well understood and there are large investments in gathering and treatment facilities. Perceived cost may be an impediment to adoption because of the need for a deep well, injection equipment with adequate capacity, and so on. However, the blind protection of sunk costs, such as what has happened with Edmonton’s expensive composting facility, is also a barrier to implementation. If a DBI site can eliminate the need for new or expanded sludge treatment facilities, the capital cost savings can be substantial (less land needed, no huge treatment ponds, etc.).

DBI does represent a complete paradigm shift for animal and municipal biosolids treatment. Agronomic and municipal engineers do not understand everything about geology, geochemistry, and petroleum technology, and few engineers and geoscientists understand agronomic and municipal technical and economic issues; therefore, there are disciplinary barriers to the adoption of such new technology. These barriers to acceptance can only be eliminated through education and careful scientific studies.

Once overall benefits and low risks of DBI are clearly understood and the unit costs for a suitable scale facility are shown to be reasonable, it should gain acceptance. A demonstration project is likely to be a key factor in that process, and the City of Los Angeles project will prove a seminal event in the evolution of biosolids disposal.

The potential environmental and economic benefits of deep injection of solid biological wastes are so large that it deserves to be studied in detail wherever there is a need and wherever suitable geological conditions exist.

Figure 1: A Slurry Injection Site in Alberta for the Disposal of Non-Hazardous Oilfield Wastes

Figure 2: The Deep Disposal Concept for Municipal and Animal Biosolids

Maurice Dusseault, GEOMEC a.s.

[1] Los Angeles will be implementing the first full-scale field implementation of DBI, and injection will start in the last half of 2004. As of Dec 2003, all permits and agreements seemed to be on place to allow this to proceed. Construction is scheduled to start in January.