Abundance

Municipal wastewater treatment plants in industrialized countries receive wastewaters from residential sources, industry, groundwater infiltration, and stormwater runoff. The pollutants associated with these sources include a wide range of suspended and dissolved compounds and oxygen-demanding materials, many of which are toxic. Pathogenic components are present, includ­ing certain bacteria, viruses, organic compounds, inorganic nutrients, and heavy metals. The purpose of most wastewater treatment processes is to remove or reduce these components, other pollutants, and biological oxygen demand before discharge to receiving waters. In the 1970s and 1980s, about 70 to 75% of the U. S. population was served by wastewater treatment facilities (U. S. Environmental Protection Agency, 1985; 1990). In 1992, more than 20,000 treatment and collection facilities served 180.6 million people or 71% of the population (U. S. Environmental Protection Agency, 1993). Of the 20,000 facili­ties, 15,613 provided treatment; the design capacity was 149 billion L/day. The need for new wastewater treatment capacity is expected to increase with growth of the sewered population. The need to treat 172 billion IVday is projected for 2012.

Primary biosolids (settleable and suspended solids) are present at a level of a few percent in the influent wastewater and are produced at a rate of about 0.091 dry kg/person-day (0.20 dry lb/person-day). Per million population, this corresponds to the production of 33,200 t/year of primary biosolids. After conventional primary and secondary wastewater treatment, the digested, dewa­tered biosolids are reduced to the equivalent of about 0.063 dry kg/person — day (0.14 dry lb/person-day), or 23,000 dry t/year per million population. For the United States in 1995, primary and treated biosolids production were about 8.6 and 5.9 million dry tonnes.

About 40 to 45% of the treated biosolids are disposed of in municipal landfills, 30% is applied to land or distributed or marketed as fertilizer, 20% is incinerated, and the remainder is disposed of in dedicated landfills or by a few other methods.

Availability

Again, there is no question of the physical availability of biosolids. They are collected in municipal wastewater systems and are therefore available in centralized locations. But in this case, treatment is essential for health reasons and protection of the public. Unless processes exist that can be used to treat and stabilize the waste and at the same time recover energy, it does not make much sense to use untreated biosolids as a waste biomass feedstock. In fact, such processes exist and will be discussed in some detail in later chapters. The other option to consider is the utilization of treated biosolids as a waste biomass.