Environmental Biotechnology - Jordening and Winter
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inorganic pollutants, as required by environmental laws and enforced by state control agencies of the respective countries. The concentration limits in the purified wastewater for residual carbon (measured as biological oxygen demand, BOD, or chemical oxygen demand, COD), nitrogen (total nitrogen or ammonia nitrogen), and phosphorous (in particular, soluble orthophosphate) that had to be met for disposal into surface waters became more stringent with time. Improvements and the development of new processes for wastewater purification were stimulated by pressure exerted by legislation.
Due to the complexity of the pollutants in different wastewater types or even in a certain wastewater of defined composition, combined multistage processes for physical, chemical, and biological removal of organic pollutants, nitrogen, and phosphorous were required. Generally, the process development for wastewater purification was always slightly ahead of an exact knowledge of the biological reactions or reaction sequences behind it. A lack of detailed biochemical knowledge on even major metabolic pathways or, in particular, on single reactions within the complex ecosystem called wastewater was always the bottleneck for specific improvements in wastewater purification techniques and treatment efficiencies, favoring a trial-and-error approach by civil engineers. Another nuisance was an apparently deep gap between the scientific approaches of civil engineers (practical process designers and operators) and life scientists (basic researchers). Even today, finding the bottleneck reactions is still one of the obstacles to improving wastewater treatment.
19.2.1
Domestic Wastewater
In the 1920s treatment of domestic wastewater began in big cities, with the construction of sewer systems and large treatment units. Later, many small wastewater treatment plants, often considered less efficient, were built in smaller settlements to serve single towns or villages in less densely populated areas. With time, development was directed more and more toward centralization of wastewater treatment, with huge treatment units serving whole regions. These units were supplied with wastewater from several settlements, often via pumping stations to transport the wastewater over long distances. This development was subsidized by the government and favored by the inspecting offices, since the treatment efficiency in these plants was considered more reliable (or easier to control) than that in a large number of scattered, local, small wastewater treatment plants. However, except for accelerating costs for additional pumping stations and the construction of new central sewage treatment plants or the extension of the capacity of existing plants, another possible source of environmental pollution was or still is created: thousands of miles of main sewers becoming leaky with age and allowing wastewater to seep into soil and groundwater. This must be a major concern, since groundwater in many regions of the world is the main source of potable water, so its pollution must be prevented.
Since all wastewater sources in settlements, including rainwater from roofs of buildings and streets, often flow abundantly into mixed-water sewer systems, the
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wastewater reaching the treatment plant during or after rainy weather is highly diluted. All wastewater treatment facilities must be designed to cope with such unfavorable conditions with respect to the hydraulics and the chemical and biological reactions. On the one hand, dilution of wastewater is counterproductive for efficient chemical or biological treatment; but on the other hand, rainwater is periodically required to flush the channels free of sediments, due to the construction of the sewer systems having little slope. In some communities or newly developing suburbs, a dual channel system for separately collecting sewage and rainwater is available.
For testing alternative wastewater handling approaches, in some new residential areas the general strategy of collecting all wastewater types into a common sewer system for treatment in a central sewage plant has meanwhile been reversed. Less polluted rainwater, e.g., is collected in natural or artificial depressions in the ground, where it seeps into the underground, with the top soil layers serving as a natural biologically active filter. Only limited paved areas are allowed, to favor the seepage of as much rainwater as possible.
Grey water from single households can be purified biologically in special soil filters planted with Phragmites australis, Typha angustifolia or other plants that develop an aerenchym. After removal of most of the pollutants by biofiltration, the purified wastewater seeps into the underground. The natural self-purification capacity of the top soil layers for wastewater components is extended into deeper layers of the soil by improving the oxygen supply via the aerenchym of planted vegetation.
To reduce the amounts of waste and wastewater, separation toilets have been developed that handle feces and urine differently. After utilization, these toilets are flushed with very little water; the solids are separated and composted in-house, and a concentrated mixture of urine, some suspended matter, and the flushing water flows through a sewer system to a nearby biogas plant for wastewater stabilization and biogas production. The ammonia of the fermented ‘yellow water’ should be recoverable for use as fertilizer.
Compact treatment units for human excrements with solids separation and longterm hydrolysis, preanoxic zone fixed-bed denitrification, and fixed-bed activated sludge treatment for carbon removal and nitrogen oxidation, followed by pasteurization for direct disposal (if disposal standards are less strict) or microfiltration and UV irradiation for reutilization of the purified wastewater have been developed on a small scale for railway passenger cars or small ships (total volume 600–900 L). On a larger scale, such units are available for single buildings to serve up to 20 inhabitant equivalents (e.g., Fa. AKW, Protec GmbH, Hirschau, Germany). Several of these units have been operated successfully for years. If recontamination of the purified and ultrafiltered wastewater can be prevented, it can be recycled for reuse in toilet flushing. If the grey water from showers and bathrooms of houses is also purified in these minisewage treatment plants, much more water than necessary for flushing toilets is generated, and no drinking water at all has to be wasted for toilets. Only little maintenance, usually once a year, is required.
At present, some pilot projects are under investigation in new residential areas in Germany and other European countries to test zero-emission concepts. These are, however, realistic only for single houses set on several acres of land or in settlements
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with sufficient area for seepage of ponded water into the underground. A double piping system to supply the kitchen, bathroom, and laundry room with high-quality drinking water and toilets with less pure, purified rain and grey water could help to save drinking water resources. Purified rainwater from the roofs of the houses might even be used for laundry and thus further reduce the drinking water demand.
In the future, increasing costs for processing and supplying drinking water and for wastewater treatment might lead to a decrease in the drinking water consumption of single households. A separation of costs for drinking water supply and wastewater treatment might favor individual on-site wastewater treatment systems for single households or small communities. Rain and grey water purification for reutilization within the household, e.g., for laundering, toilet flushing, or watering the garden, may still sound futuristic (and to some people not acceptable), but may become necessary in the future, especially when not enough fresh water is accessible during dry seasons. First indications of a shortage of fresh water became apparent in the very dry year 2003 even in Germany, when the water level in Lake Constanz, the main drinking water reservoir in southern Germany, unusually fell several meters.
19.2.2
Industrial Wastewater
Biological wastewater treatment in industry has focused on three approaches: Some companies favor a central treatment plant for all production units, requiring the enrichment of an ‘omnipotent’ microbial population at suboptimal loading. Other companies favor smaller treatment units for each production process, requiring the enrichment of specialized bacteria in each plant, which could then be operated at maximal loading. Mainly known from the paper recycling industry are partial process water cleaning procedures for process water recycling. Only a small part of the total process water is released for final wastewater treatment. Whereas the mechanically purified recycling water should be biologically inactive (which in practice is achieved by adding biocides), the disposed wastewater must be biodegradable by microorganisms, which demands nontoxic biocide concentrations.
Wastewater purification for disposal into a natural water source always requires a technically sophisticated combination of mechanical, chemical, and biological processes essential to purify the multiple components contained in waste fluids from production processes.
If the wastewater from certain production processes contains xenobiotic substances, and these substances cannot be adsorbed, precipitated, or biologically degraded, environmental protection has to go one step further and intervene in the production process. The process must be altered to prevent nondegradable xenobiotics from appearing in the wastewater or, if this is not possible, their concentrations must be reduced to the minimum and single wastewater streams should be recycled internally to avoid environmental pollution.
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19.2.3
Effluent Quality and Future Improvements
Every wastewater that has been purified according to present treatment standards still contains some residual pollutants, consisting of a small proportion of BOD (biodegradable, but residing organic substances) and a higher proportion of nondegradable COD (organic substances that resist rapid biological degradation and require more drastic conditions for chemical oxidation), as well as a certain salt load. Further elimination of some or most of these compounds and of the suspended residual bacteria can be achieved by modern membrane technologies, such as reversed osmosis or ultrafiltration. These are, however, not yet widely used for wastewater treatment, and experience with their long-term performances is still rather scarce. They must, however, be used if the pathogen content of treated wastewater – a potential source of infections or epidemics – must be reduced to much lower concentrations than obtained by conventional sedimentation or soil filtration.
The residual BOD and part of the COD of the purified wastewater are degraded in the receiving water and some of the salt components may be precipitated. However, even if the biological self-purification capacity of receiving waters is not exceeded, traces of nondegradable wastewater components, such as detergents, household chemicals, antibiotics, pharmaceuticals, pesticides, and fungicides, are washed into the groundwater. Some of these substances have hormonal activity and act as endocrine disruptors. They influence the reproduction of wild fauna and may damage the flora in the receiving lakes and rivers or exert a negative effect on human health. If the raw water for the preparation of drinking water contains such contaminants, they must be carefully separated, e.g., by adsorption onto charcoal. Alternatively, membrane technologies can be used to separate large molecules of trace pollutants from the bulk of the water.
Future wastewater handling must more and more begin at the sources of waste and get away from an almost exclusive end-of-pipe-treatment, as has been practiced to date. Two main goals have to be envisioned:
•reduction of the total amount of wastewater by water-saving and water-recycling procedures
•reduction or avoidance of biologically nondegradable chemicals as water polluting ingredients
The first goal requires strict in-house or in-factory water-saving regimes and recycling techniques, and the second goal, a change in production processes and human habits.
Complete closure of the water cycle is already achieved in some industries, e.g., the paper recycling industry. However, new problems appear with recycling of production water. Massive germinationof the water during interim storage and reutilization requires permanent application of biocides. Formation of fatty acids from carbohydrates by contaminating microorganisms has meant that only low-quality papers could be produced from the recycled cellulose material. If these papers are
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moistened, an unpleasant odor develops; in addition, their acidity favors rapid deterioration.
To promote environmentally sound production processes in industry, the socalled Ökoaudit system was introduced by environment controlling authorities in Germany. Input material, products, and wastes are analyzed, and improvements in working procedures or production processes are proceeded by the management of the company. With their participation in the Ökoaudit evaluation, a company agrees to reduce pollution year by year by a certain percentage compared to the present state. To make consumers aware that a company uses an environmentally friendly production process (which may justify a somewhat higher retail price), the participating companies are allowed to print a symbol of environmentally friendly production on their products.
19.3
Solid Waste Handling
Until only recently, solid domestic wastes and residues from industrial production have been collected and simply deposited into sanitary landfills located outside residential areas. Except for sorting out most of the recyclable material and a certain degree of homogenization, no other treatment was considered necessary. Only highly poisonous industrial wastes were deposited underground in abandoned salt mines, mining caves, or tunnels.
Most waste pretreatment or treatment procedures, other than just deposition in sanitary landfills, were developed during the last two to four decades. Recently, a new deposition guideline of the European Community (EU-Deponierichtlinie 1999/31/EG, European Commission, 26 April 1999) was introduced that defines the construction, operational, and aftercare requirements of the three exclusively allowed sanitary landfill classes of the future for either:
•dangerous industrial wastes
•nondangerous wastes such as domestic refuse
•inert monowaste material without chemical or biological reactivity
The EU guideline prescribes a leak-proof construction of bottom and top seals and focuses on deposition techniques, whereas the German technical instruction for the handling of domestic wastes (TA Siedlungabfall and Abfallablagerungsverordnung AbfAblV 2001) is much more stringent. In addition to comparably detailed prescriptions regarding the construction of sanitary landfills, the TA Siedlungsabfall defines a maximum organic dry matter content of 3% or 5% of the wastes for deposition into class I or class II sanitary landfills, to guarantee an inert or quasi-inert behavior after deposition. Class III landfills are monodeposits for certain inert, nondangerous waste materials.
The German federal waste recycling law (Kreislaufwirtschaftsgesetz-KrW/AbfG 1996) defines three top priorities:
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•avoidance of wastes: to reduce the amount of waste material to a minimum
•recycling of wastes:
–as secondary raw material (substance recycling)
–as a source of energy (energy recycling)
•deposition of wastes
Deposition of untreated wastes is restricted until 2005. After then, only deposition of fully inert wastes such as incineration slag or ash will be allowed according to the environmental law. However, due to a shortage of incineration capacity, mechanically and biologically pretreated wastes with low residual respiratory activity or methane production capacity and a high lignin-to-carbohydrate coefficient may be deposited until 2025, according to an approved amendment to the German environmental legislation.
Whereas avoidance of wastes, especially of packaging wastes, still seems to have a real potential for improvement, recycling of waste materials from other waste types has reached a high overall level. For example, glass recycling is characterized by high recovery rates, presumably because the quality of glass products made from recycled raw materials is almost the same as that of virgin glass.
Paper recycling by standardized procedures is also well introduced and accepted. However, due to breakage of fibers with every round of recycling, paper recycling is more a process of downcycling. A certain percentage of fresh fiber material must be added to achieve constant product quality. Paper from recycled raw material has to compete with paper made from low-quality wood or waste wood, which is available in high quantities in the Northern, wood-producing countries of the world.
Whether plastic material should be recycled for the production of new plastic goods is a matter of discussion. To achieve high qualities of recycled-plastic products, plastic wastes have to be cleaned of contaminants. Then the mixed plastic material must be separated into the different polymer fractions (which apparently is not efficient with the procedures now available) for specific recycling of each polymer class. It may be more favorable to use mixed plastic wastes for energy recycling, e.g., using mixed waste plastic as fuel in the production of new plastic materials from the fossil petroleum saved by not using the latter as fuel.
To achieve the low carbon content of 5% or even 3% of organic dry matter content, as required for deposition, the nonrecyclable fractions of municipal or industrial wastes must be incinerated. Pyrolysis alone does not suffice. Waste incineration leads to two main residual products: gas and slag. Both are highly polluted with toxic materials, but their concentrations of toxins are not as high as in fly ash. Whereas purification of the off-gas from waste incineration can be considered a state-of-the- art process enforced by various state laws, e.g., the 17th German Federal Ordinance on Protection from Emissions (17. BImSchV 1990), disposal or proper utilization of the ash and slag is still a matter of controversy. Incineration slag has been used to construct traffic noise protection barriers along highways. Long-term reactions of the heavy metal oxides might, however, lead to their remobilization and cause environmental harm.
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After removal of the powder fraction of slag by sieving, the granular fraction can be used as a raw material in the construction industry. However, there seem to be gaseous organic inclusions in the slag which slowly diffuse out of concrete walls of houses and harm the inhabitants. In addition, some residual toxic organics remain in the ash, even when the waste incineration efficiency is high.
To obtain less toxic incineration residues in the future, detoxification of slag and fly ash is considered not only a desirable option, but a requirement – even if it seems too expensive now. In Switzerland, a new technique for separating toxic heavy metals in highly toxic filter ash from waste incineration plants from nontoxic mineral products was tested on a laboratory scale (Chemische Rundschau No. 15, 1998). The heavy metal ions react with hydrochloric acid to form their chlorides, which can be evaporated at 900 °C. This would avoid deposition of toxic fly ash in mining shafts or solidification of the toxic material with concrete. To save energy, this treatment should ideally start with still-hot ashes.
19.4
Off-gas Purification
For removal of organic and inorganic pollutants from huge quantities of highly polluted waste gas streams from, e.g., the lime and cement industry, coal-fired power plants, or waste incineration plants, technical procedures are available for the separation of fly ash by gas cyclones, particle filtration or electrofiltration; the removal of acid and alkaline gas impurities by washing procedures; and the removal of neutral trace-gas components by washing/adsorption/gas filtration. Off-gases from composting plants, pork and chicken breeding facilities, etc., which are mainly polluted with volatile organic compounds, can be purified by gas washing and aerobic/anaerobic treatment of the washing water in biofilters. Natural and synthetic filter materials have been used as support materials for the development of active biofilms on biogas or off-gas filter units, which in principle resemble the trickling filters used in sewage treatment. To maintain a permanent high adsorption and degradation efficiency, the moisture content of the filter material must be kept high enough to keep the biofilm in an active state, and addition of trace elements may eventually be required to support optimal growth and metabolic activity of the microorganisms forming the biofilm.
19.5
Soil Remediation
Due to an almost unlimited number of pollutants and to different soil and underground structures, no general guideline for soil remediation is possible. Since soil is an agglomerate of mineral compounds, including small particulate materials such as clay, gravel, and stones, and – at least in the upper layers – of organic materials (e.g., plant residues, organic fertilizers, humic substances) and its adsorption capac-
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ity for toxicants changes at different moisture contents or water conductivities, the retention of hydrophilic, water-soluble and hydrophobic, water-insoluble contaminants in soils varies. In particular, water-insoluble light compounds (NAPL = nonaqueous phase liquids) tend to accumulate on top of the groundwater level.
Besides safeguarding contaminated sites by, e.g., inertization and encapsulation, several methods for decontamination of highly polluted soils and groundwater at former industrial sites have been practiced in the past, including ex situ soil treatment (incineration, soil washing and soil extraction procedures, land farming, and biological cleaning) and in situ remediation (bioventing or biosparging, natural and enhanced natural soil remediation). Ex situ treatment is easier to control but expensive, whereas in situ remediation is less easy to control and becomes less efficient with time but is much cheaper.
In locally restricted soil compartments that have been contaminated with highly toxic chemicals or metals (e.g., mercury), excavation and thermal or chemical treatment (incineration or chemical extraction of the pollutants) may be necessary to restrain spreading of the toxic contaminants or of toxic metabolites during sanitation. By excavation and, e.g., thermal soil treatment, not only is the original soil texture destroyed, but also the redox state is shifted toward oxidized sinter products. However, a high decontamination efficiency even of micropores can be obtained within a short treatment time. If the contaminants are highly volatile, on-site decontamination should be favored, since otherwise, extensive precautions for transport in closed containers are necessary, as is true for soil contaminated, e.g., with poisonous solvents or leaded antiknock agents.
Except for those sites with highly toxic contaminants requiring ex situ treatment procedures, the more economical but eventually less quantitative in situ procedures could be used in many contaminated sites. The procedures include soil stripping with solvents or water and groundwater recirculation after purification of the stripped liquid (hydraulic procedures) or biosparging (gas venting through groundwater and the unsaturated soil), purification of the ventilating gasses from the contaminants, and reintroduction of the purified gases. If a light liquid phase is floating on the surface of the groundwater, the bioslurping procedure may be the most appropriate means for remediation; bioslurping is a combination of stimulation of biological processes and contaminant removal. Direct in situ bioremediation techniques are suitable for soils and groundwater with high liquid and gas conductivity, if biodegradation of the contaminants can be achieved by air injection, mineral or carbon addition, or groundwater recirculation. For in situ bioremediation, until now only a limited number of possible techniques have been developed for practical application, mainly because the efficiency of biological in situ removal of contaminants is limited by several factors concerning the contaminants (e.g., low solubility, strong sorption onto the soil matrix, diffusion into macropores of soils and sediments) and concerning the transport of nutrients and electron acceptors for microbial activity (e.g., permeability and porosity of the soil, its ion exchange capacity, pH, and redox potential).
The least invasive in situ remediation approach is based on the identification of intrinsic bioremediation factors, to show that under suitable environmental condi-