Wastewater Treatment

Historically, wastewater processing qualifies as our first approach to using applied biology constructively on an environmental basis. Natural biological processes have, in fact, dealt with the world's various waste streams from the earliest days of microbial life eons ago, but our own efforts with respect to waste management were barely better than those of ants until well beyond the Renaissance, content as we were with primitive privies, latrines, and pits for mere containment rather than complete treatment (Asimov, 1989).

Remarkably, our initial motivation to use applied waste treatment roughly five centuries ago was neither that of resolving serious and escalating health concerns tied to disease transmission nor that of reducing widespread aesthetic degradation. Instead, our original goal was actually that of commercial manufacturing, whereby our residuals might be transformed from disagreeable wastes into truly useful products with monetary value.

The classic set of sixteenth-century lithograph prints shown in Figures 16.1 and 16.2 depict two such waste processing systems, located in England and Germany, respectively, both designed in a fashion that we might presently qualify as biological nitrification filters. Urine and other nitrogen-rich wastes (e.g., slaughterhouse effluents) were pumped or ladled onto the filters and allowed to trickle over and through the piles. In turn, nitrate-rich

Figure 16.1 Sixteenth-century English nitre farming.
Figure 16.2 Sixteenth-century German nitre farming.

salt crystals (i.e., long know to alchemists as nitre) would then be scraped from these filter surfaces and sold as a key ingredient for producing gunpowder.

These commercial operations predated the discovery of bacterial life by several centuries. It was not until the late nineteenth century that we finally understood the true litho-trophic basis for reliance on biochemical nitrification to convert reduced ammonia to oxidized nitrates. Despite this fundamental uncertainty, nitre pile waste processes continued to represent a significant chemical manufacturing industry for several centuries and was even used for critical nitrate production during the Civil War by Confederate states in the southern United States.

Midway through the nineteenth century, a number of larger English cities began to use a patented processing scheme to recover waste nitrogen, but in this case the desired product was a nutrient-rich fertilizer. These so-called "ABC" systems used an altogether unusual set of chemical additives, including alum, blood, and clay (i.e., hence the unique acronym) to clot and sorb a wastewater's unwholesome contaminants while generating a nitrogen-bearing agricultural fertilizer sold under the promotional trade name "native guano.''

At about the same time, securing complementary environmental benefits with waste-water treatment began to reach beyond the product-oriented stage, catalyzed both by the latest discoveries in microbiology and medicine and a belated recognition of the inherent necessity for sanitary and environmental improvements. These early years in the field of wastewater treatment were exciting times for this fledgling niche of applied environmental biology.

Land application operations were the earliest large-scale systems used for wastewater treatment, with several such operations being in use in Europe. The first land-application "filters" were remote tracts of land onto which wastewater would be spread, being cleansed as it passed through the soil. As our understanding of, and appreciation for, the underlying microbial mechanisms evolved during the middle to late nineteenth century, simple in-ground filters were soon followed by a steady progression of ever more advanced, and more heavily loaded, intermittent soil filter, contact bed, and trickling filter wastewater systems. In each case, the soil, gravel, rock, slate, and slag media surfaces involved were found to be densely covered with an attached biofilm rich in waste-degrading biomass, following a wastewater processing mode now classified as attached-growth treatment.

Early in the twentieth century, another seminal discovery was made that coagulated and settled biomass suspensions such as those derived from either settled discharges taken from the early attached-growth biofilm reactors or from even newer suspended-growth (as opposed to attached-growth) biomass tanks, could actually be recycled back to an aeration chamber for subsequent reuse, thereby accelerating the overall metabolic efficacy of these biological wastewater processing systems. Largely developed at Manchester, England, by Sir Gilbert John Fowler in the period 1912-1915, with several large full-scale facilities being built around the world within a period of just a few years, this newly devised activated sludge concept rapidly evolved into yet another parallel mode of suspended-growth system, further escalating the sophistication and effectiveness of wastewater treatment (Horan, 1990).

In the twenty-first century, the applied science of biological wastewater treatment continues to evolve in terms of the level of technical sophistication employed. Attached- and suspended-growth options still comprise the two major options for biological treatment, but in either case these older technologies have been upgraded in technical complexity and hardware, including all manner of internal pumping schemes, advanced aerators, online instrumentation, and computer-based control and automation. Whatever the approach, these wastewater processing systems now arguably represent the largest controlled application of microbiology in the world, at a scale far outstripping that of commercial and industrial fermentation and pharmaceutical production.

Whether based on attached- or suspended-growth processing methods, the overall technology of wastewater treatment (i.e., as applied in large-scale fashion by cities and industries rather than that of small-scale septic tanks, etc.) includes an integrated collection of processing steps encompassing not only biological processes, but also a complementary set of physical and chemical mechanisms. Figure 16.3 depicts the processing steps used with these types of conventional four-step systems. The initial preliminary treatment step screens, settles, and separates coarse solids, and a primary treatment step provides further solids separation, either by floating or settling. Primary treatment may remove onehalf to two-thirds of the incoming suspended solids and perhaps half as much of the biochemical oxygen demand (BOD), but neither preliminary nor primary treatment is able to remove colloidal or dissolved contaminants.

A subsequent secondary treatment step is used to remove the remaining biodegradable contaminants and suspended solids. As reviewed in the following sections, secondary treatment is typically completed using one of four typical processing options, including attached-growth (e.g., trickling filters) and suspended-growth (e.g., activated sludge) systems, stabilization lagoons, and constructed wetlands.

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