Solid Waste & Recycling

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Cover Story: A solid approach to control the content of sludge…What's It?

Effective wastewater treatment remains a priority on the wish lists of many Canadian environmentalists. But this wish remains unfulfilled as the water bodies downstream from several major Canadian cit...


Effective wastewater treatment remains a priority on the wish lists of many Canadian environmentalists. But this wish remains unfulfilled as the water bodies downstream from several major Canadian cities (such as Victoria, British Columbia and St. John’s and Halifax, Nova Scotia) continue to receive untreated sewage.

The ingredients of this noxious soup include human waste along with other unsavoury items flushed down toilets and washed down drains, such as pharmaceutical products. They also include effluents from business and industrial operations and, in many cases where sanitary and storm sewers are combined, runoff from urban areas. The latter two sources may contribute heavy metals, pesticides (which includes a whole host of constituent compounds), polyaromatic hydrocarbons (PAHs), and other organic compounds.

Currently, Canadian cities and towns discharge well over one trillion litres of untreated sewage annually. Simply stated and without equivocation, many cites have a dismal wastewater management track record — a hidden plight that is in many cases only worsening as municipal purses tighten under ever-increasing financial burdens.

However, there is some good news. On October 29th, 2001 Halifax Regional Environmental Partnership (United Water) was selected to enter into negotiations with Halifax Regional Municipality to undertake a $262.4-million project to design, build and operate three wastewater plants and a separate sewage collection system (which involves tunnelling in downtown areas). This project will finally address the 180 million litres of raw sewage that Halifax discharges into the harbour each day.

But before we sound a cheer for Halifax let us pose a hypothetical question. When Halifax — and eventually when every Canadian city — has truly effective sewage treatment, what will they have really achieved? While they will have undoubtedly cleaned up their water discharges, they will have achieved the cleanup by transferring the waste problem to another medium (which as we will see, Halifax has fully considered in designing its sewage treatment system). That is, they will have taken sewage and converted it into hundreds of thousands of tonnes of sewage sludge — a material whose management is the subject of an increasingly hot debate (see “Sludge Fight” in the December/January edition).

Life-cycle approach

The best bet to reduce the environmental burden of the sewage “life-cycle” as well as environmental impacts of land application is a combination of upstream control over what is allowed to become part of the sewage sludge with downstream best practices to manage final disposition as a land-applied “biosolid.”

Life-cycle inventories rely on a simple accounting of physical units or, as engineers refer to them, “mass balance.” For instance, when dental amalgams containing mercury are used, the bulk of the mercury stays within the amalgam (the filling), a very small portion is ingested by the patient (some is passed), and the remainder is washed down the drain when the patient washes their mouth out. The portion passed and the portion washed down the drain end up at the sewage treatment plant of which some remains in the sewage sludge and the balance is discharged to the receiving water body.

With a heavy-metals management program at the sewage plant being prohibitively expensive to build and operate (and according to experts in the field, technologically unproven), the simple solution becomes pollution prevention at the dentist’s office. That is, the substitution of the use of mercury based dental amalgams with the use of composite materials. Such a substitution would likely come about as dentists began to meet the stringent requirements of municipal “Sewer Use By-laws” such as the one originally proposed by the City of Toronto in 1999 under powers granted to it through Ontario’s Municipal Act.

An April 2001 literature review by R.V Anderson Associates Limited states that in “…the absence of detrimental effects in the studies with high metal concentrations and application rates, it appears that the recommended land application practices in Ontario present no significant risk to humans and the environment.”

Irrespective of whatever is subjectively considered “acceptable” for land application, the ongoing reduction of metals in sewer loadings can only increase public confidence in the practice of regulated land application of biosolids. Of even greater importance, reducing the loadings of these constituents to sewers means reducing the portion going to receiving water bodies.

Laws work. In the past several decades the introduction and enforcement of federal and provincial discharge laws has had a meaningful positive impact on water quality in the Great Lakes. Sewer discharge by-laws enable local action and bring focused water quality policies to bear on businesses and industries.

Once industrial discharges have been dealt with we might turn our attention to jurisdictions where sanitary sewers and storm sewers are combined. In normal operation these systems combine urban runoff with sanitary sewage that is then directed to the sewage treatment plant. When heavy rains occur the system overflows, sending raw sewage and urban runoff into receiving water bodies.

Urban runoff is comprised of rainwater and a host of chemicals that wash off of parking lots, industrial parks and roads. These include wearing automobile parts (chromium, cadmium, copper, lead, and arsenic), automotive emissions (PAHs, used motor oil, spilled diesel and coolant), pesticides and herbicides, and solvents and coatings (from painting and metal processing).

Barring a complete separation of the sanitary and storm sewer systems — something that may be prohibitively expensive in a large highly developed city such as Vancouver or Toronto — the next best option to reduce loadings is to direct storm-water runoff through separate settling ponds. Once interred the matter can be effectively treated through a combination of simple physical and biological processes. In so far as pesticide and herbicides are concerned, municipal restrictions on their use offers not only distinct benefits in terms of reducing loadings to the sewer system (and their subsequent incorporation into sewer sludge) but in terms of reducing overall exposure to the public.

Having taken these upstream steps, the resulting sewage sludge is freer of many of the heavy metals and organic compounds than it would have been otherwise. Nevertheless it remains noxious.

Untreated human waste contains bacteria, viruses and other pathogens that must be treated fully to render the resulting biosolid virtually pathogen inert.

Sewage sludge management currently falls into two classes of treatment: Class B and Class A.

Class B biosolids are those produced at lower cost and with less rigorous effort with the respect to the elimination of pathogens. Their production relies largely on anaerobic digestion in the wastewater treatment process to reduce the amount of solids present. In some cases treatment of undigested sludge with alkali (e.g. lime) is also undertaken. Class B biosolids are much like animal manure in both appearance and physical handling properties. And, like most animal manure, they may also contain pathogens in potentially harmful quantities. Class B biosolids comprise an overwhelming portion of biosolids seeking land application in Canada.

Class A treatment involves a multi-step effort to completely destroy pathogens. The process employed on behalf of the City of Sarnia in Ontario (very similar to the one that will be used in Halifax, Nova Scotia) is a good example. The process involves sludge dewatering, the application of an alkaline admixture (usually waste lime kiln or cement kiln dust) to eliminate pathogens, heat drying and acidity control to promote the growth of microflora that inhibit the re-growth of pathogens (and immobilize many of the remaining heavy metals).

Class A biosolids must be virtually pathogen free to be deemed as such. Though more expensive to process, the promotion of advanced p

rocesses to render sewage sludge into Class A biosolids is gaining popularity with state and provincial governments, municipalities and private operators as they seek to gain the confidence of host land application sites and their respective local communities.

The control of what enters the wastewater stream and aggressive efforts to provide pathogen-free biosolids will mean an improvement in water quality and in the quality of land applied biosolids as well as peace of mind for residents. All it takes is a concerted effort by provincial and municipal governments to put the right policies in place and enforce them in practice.

Usman Valiante is principal of General Science Works, based in Toronto, Ontario.


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