Waste recycling technologies and fertilizer production from sewage sludge

Even with sufficient areas for new landfills, their very system is unsustainable. As a result, humanity may end up with a landscape covered in "pyramids" of waste and hundreds of thousands of people servicing these landfills.

A solution to this situation could be waste recycling. There are many methods for recycling various types of waste. Let us name the most widely used technologies: - waste paper is shredded into paper pulp, from which various paper products are manufactured; - glass is crushed, melted, and made into new containers, or crushed and used instead of gravel or sand in the production of concrete and asphalt; - plastic is melted down to produce "synthetic wood," which is resistant to biodegradation and possesses tremendous potential as a material for various fences, decking, posts, railings, and other outdoor structures; - metals are melted and processed into various parts – this saves up to 90 % of the electricity required for smelting metals from ore; - food waste and garden waste are composted to produce organic fertilizer; - textiles are shredded and used to strengthen recycled paper products; - old tires are melted down to manufacture new rubber goods.

In addition to these, there are hundreds of other industrial methods for recycling waste.

Practically 30 to 50 % of the organic matter present in sewage ends up in raw sludge, which settles in sedimentation tanks and at other stages of treatment. It is a thick, black, foul-smelling mass consisting of approximately 98 % water and 2 % organic matter, containing many pathogenic organisms. After appropriate treatment, humus can be obtained from it and used as fertilizer.

Sludge treatment is based on feeding it to bacteria and other detritivores. This can happen in two ways: - in the absence of air – anaerobic digestion. - in the presence of air – composting; I. Anaerobic digestion.

Raw sludge is placed into large airtight tanks. In the absence of oxygen, bacteria feed on the sludge (anaerobic digestion), producing biogas as a by-product. It contains carbon dioxide and substances that give sewage a foul odor, but it consists of almost 60 % methane. The latter circumstance makes it possible to use biogas as fuel. In practice, it is used to heat the tanks themselves in order to maintain an optimal temperature of about 38°C for the organisms.

Digestion is completed in 4-6 weeks, and treated sludge – an aqueous solution of humus – remains in the tanks. This solution can be used to fertilize agricultural fields and lawns in liquid form, as both the humus and the water rich in nutrients are beneficial. Treated sludge can be filtered to obtain a semi-solid humus cake, although the main portion of nutrients is lost with the filtered water, which reduces the nutritional value of the cake. II. Composting.

For composting, raw sludge is filtered, mixed with wood shavings or other material to improve aeration, and piled into heaps or compost rows. Aeration is increased by additional air supply or mechanical mixing. In compost piles, bacteria and other reducers and detritivores process the organic matter into a humus-like mass. The heat generated during respiration is sufficient to kill pathogenic organisms.

After six to eight weeks of composting, the humus is separated from the wood shavings and is ready for application in the fields.

In recent years, the co-composting of municipal solid waste and sewage sludge has been increasingly developed. This technology contributes to the enrichment of compost with microflora and micronutrients and allows for the maintenance of the biothermal process in an optimal mode. It is accompanied by heating the mass to 60–70°C. During this process, most pathogenic microorganisms, helminth eggs, and fly larvae are killed.

Incineration of solid waste is expedient if heat energy is utilized and exhaust gases are treated. This process takes place at waste-to-energy plants that have steam boilers with special furnaces. The temperature in the furnace must be at least 1000°C so that all foul-smelling gas impurities are burned and slagging of the grates does not occur. Before exiting into the chimney, gases are cleaned, for example, using electric filters. Metal scrap is separated from the slag by an electromagnetic separator. Other non-combustible residues require landfilling, but they account for only 10-20 % of the initial waste volume.

The use of all the methods for reducing environmental pollution discussed in this chapter does not fully solve the problem and is associated with increased costs for their implementation. An alternative is the introduction of waste-free and low-waste production processes.

Zero-waste production can be understood as a set of technological processes in which the waste from one process is used as raw material for another, which ensures its almost complete utilization. For example, ash generated from burning organic fuel can be used in the production of silicate bricks, as a concrete filler, etc.

Creating zero-waste production is a very complex and lengthy process, an intermediate stage of which is low-waste production. In low-waste production, the environmental impact does not exceed the level established by sanitary and hygienic standards. At the same time, for various reasons (technical, economic, organizational, etc.), some raw materials and substances may turn into waste and be sent for storage or disposal.

Low-waste and zero-waste technologies must ensure: - complex processing of raw materials using all their components; - creation and release of new types of products taking into account the requirements for their reuse; - processing of production and consumption waste to obtain commercial products or any other beneficial use without disturbing the ecological balance; - use of closed industrial water supply systems; - creation of zero-waste complexes.

Stages of development of low-waste and zero-waste production: 1) low resource intensity and insignificant emissions into the environment; 2) creation of production cyclicity – the waste of some processes serves as raw material for others; 3) organization of rational disposal of unavoidable residues and

All stages can be simultaneous. As a result, economic production costs are reduced, the complexity of raw material use is achieved, and the problems of reducing environmental pollution by waste are solved more effectively.

Human production and other activities lead not only to the chemical pollution of the biosphere discussed in previous chapters. Physical pollution is playing an increasingly important role in the overall flow of negative anthropogenic impacts on the biosphere. The latter is associated with a change in the physical parameters of the external (surrounding) environment, that is, with their deviation from the parameters of the natural background.

Currently, the greatest attention is paid to changes in electromagnetic and vibro-acoustic parameters (conditions) of the environment. As a rule, in literature, they are considered as wave or energy pollution.

The frequency spectrum of electromagnetic oscillations known today is extremely wide: from near zero to 3·1022 Hz (X-ray radiation). Due to this circumstance and the difference in methods of generation and registration, as well as the diversity of manifestations of electromagnetic oscillations, the entire spectrum is divided into several bands.

1. Radio waves, excited by the movement of electric charges in systems formed by bodies of macroscopic (supermolecular) sizes, frequencies 0 6·1012 Hz 5·10-5 m, according to international radio regulations, radio wave lengths (frequencies) are divided into 12 bands, starting from extremely low frequencies 3···30 Hz, ending with hyper-high frequencies 0.3···3 THz.

2. Optical waves (radiation), excited by the movement of electric charges in atomic-molecular size systems. The frequency spectrum, f = 3⋅1011···3⋅1016 Hz (the boundaries are conditional), λ = 10-3···10-8 m. The entire optical radiation spectrum is divided into three bands: - infrared, f = 3⋅1011···3.9·1014 Hz, λ = 10-3... 0.77·10-6 m, or thermal radiation; - visible, f = 3.9⋅1014···∼7.9⋅1014 Hz, λ = 0.77⋅10-6···0.38⋅10-6 m, or light radiation; - ultraviolet radiation, f = 7.9⋅1014···3⋅1016 Hz, λ = 0.38⋅10-6···10-8 m.

3. X-ray radiation, occurs during the interaction of charged particles and photons with atoms of matter, f = 3⋅1015···3⋅1022 Hz, λ = 10-8···10-14 m.

4. Gamma radiation, generated by excited atomic nuclei during radioactive transformations and nuclear reactions, during particle decay, etc. 3⋅1018 Hz 10-10 m.

It is generally recognized that at present, the greatest contribution to the energy pollution of the environment is made by changes in its electromagnetic parameters in frequency ranges corresponding to the regions of radio waves (proper electromagnetic pollution), infrared or thermal radiation (thermal pollution), X-ray and gamma radiation, which, together with α- and β-particles (emitted by radionuclides – unstable nuclei of radioactive elements: uranium, thorium, polonium, radium, etc.), are the cause of radioactive pollution of the biosphere, as well as changes in vibro-acoustic parameters (vibro-acoustic pollution).

One of the fundamental components of the complex of measures for protecting the environment from energy pollution is their rationing, that is, establishing the level of energy pollution, the exceeding of which is unacceptable when organizing a new production facility (factory, thermal power plant, etc.) or reconstructing an existing one in a given location. If for chemical pollution this threshold level is the maximum permissible concentration, MPC (see section 7.5.), then for energy pollution, a maximum permissible level, MPL, of energy pollution has been introduced. Its meaning corresponds to the meaning of MPC. As in the case of MPC, MPL is established separately for the work zone and for the environment (populated area). The latter is always lower than the MPL for the work zone. In a large number of cases, this difference is 10 times, which can be explained by two circumstances. Firstly, the work zone, i.e., production facilities, is occupied by, to use ecological-biological language, mature (physically and biologically) human individuals. Their resistance to the impact of harmful factors (tolerance range) is higher than that of other parts of the population: children, the elderly, and those who are simply physically weak. Secondly, to a significant extent, this difference is predetermined by the fact that in most cases, the harmful factor is formed specifically in the work zone, where it has maximum values; as it spreads in the surrounding space, its intensity decreases, so that outside the production territory, its intensity is a priori lower than in the work zone. It turns out that to a certain extent, this dual rationing merely records the objective distribution of the intensity of a harmful factor in space (this applies to both chemical and energy pollution).

Of course, economic considerations lie deep at the heart of today's regulation of harmful environmental factors. The reader will immediately agree that it is best to ensure that both the concentration of harmful substances and the intensity of energy factors are negligibly small. Alas, this is the flip side of technical progress and the root cause of the Global environmental crisis: it is not always possible to design a production technology in such a way as to completely eliminate its harmful impacts.

The market is, in principle, anti-ecological. A market-driven producer must either invest heavily in refining technology to eliminate harmful effects, in which case their "train may have left the station," or they break into the market with their product (or service), leaving a trail of environmental costs behind. Yet, this trail of environmental costs (clearly harmful, and by all human standards, needing to be eliminated!) is agreed upon (!) with regulatory and environmental oversight bodies based on the compromise concept of MPC (Maximum Permissible Concentration) and MPL (Maximum Permissible Level), and soon the product is circulating the globe, contributing to that planetary phenomenon we now call the Global environmental crisis.

In principle, however, the compromise concept of MPC (maximum permissible concentrations) and MPL (maximum permissible levels) is today a real and effective tool for environmental protection. And it will find its place in the process of implementing the sustainable development concept. But it will find its constructive applications only in development.

The direction of its development: the levels of MPC and MPL must be reduced. Life, that is, the practice of implementing the sustainable development concept, will suggest the pace at which environmental standards should be tightened. Naturally, it must be optimal: neither excessively fast nor too slow. It will be whatever the Collective Intelligence of the world community determines, with constant consideration for the pace of the Global environmental crisis.

But apparently, the matter will not consist solely in the constant tightening of environmental standards in their current form. Most likely, the philosophy (criteria) of the environmental standards themselves will also change, in particular, the standards for energy pollution.

In accordance with modern ideas about the interaction of an energy factor with an organism, one can outline the following logic for defining (searching for) the MPL. For example, in relation to the acoustic (noise) factor.

Having undergone a long evolutionary process, humans have adapted to the real spectrum of acoustic impacts. During evolution, it proved disadvantageous for the human auditory system to register sound signals with an acoustic energy flux density lower than 10-12 W/m2 (the zone of preventive inhibition): apparently, all phenomena of the surrounding world accompanied by the emission of sound signals with an energy flux density lower than 10-12 W/m2 (threshold sensitivity) did not play a significant role in the vital activity of our prehistoric ancestors and therefore were instinctively "cut out."

More energy-intensive signals carried more significant information; as a result, our auditory system perceives sound signals in an extremely wide range of energy flux densities, which forms the information zone. All sounds in this zone helped prehistoric humans (and help us) to navigate the surrounding world.

The right (upper) boundary of this range of sound energy flux densities, and higher values thereof, apparently corresponded to those real sound signals that also did not genuinely carry significant information (otherwise the auditory system would have adapted to them). This is the zone of transcendental inhibition, as these signals cannot be specifically perceived, since they disrupt the normal self-regulation of the organism, and the latter is forced to defend itself against their action through protective inhibition.

It is natural to take as the upper boundary of the information zone a value comparable to the basal metabolic energy of the organism, recalculated as an equivalent of energy flux density. The basal metabolic energy, practically the same for all warm-blooded livestock animals, is:

If we express this value in decibels relative to the threshold of sensitivity

L = 10Lg −12 ≅ 107dB, (9.1) we obtain the average sound pressure level at the upper boundary of the information zone. It corresponds to the basal metabolic energy in a state between rest and intense muscular work. In the latter case, the basal metabolic energy can increase by 5-8 times, that is, by approximately an order of magnitude. For this case, L = 117 dB ≅ 120 dB. Accordingly, for the state of rest, L ≅ 100 dB. At the same time, this is the lower boundary of the zone of transcendental inhibition, or the zone of energy impact.

Thus, the magnitude of the basal metabolic rate, or more precisely, the spectrum of values of this energy corresponding to the spectrum of organism states between rest and intense physical work (in decibels, this spectrum is expressed as 100···120 dB), can be considered as the initial value when establishing theoretical maximum permissible levels (MPLs) of energy pollution (exposures). Furthermore, this can be applied to every energy exposure: in the course of evolution, organisms have adapted to all energy exposures; for each of them, organisms possess an information zone and, accordingly, the upper boundary of this zone, which is comparable to the basal metabolic rate. It is therefore natural, as a certain approximation of the MPL for one energy exposure or another, to consider its parameters that correspond to the energetics of the basal metabolic rate.

Ecological Standardization of Energy Exposures to the Biosphere

When using sewage sludge as fertilizer, it is necessary to consider the level not only of chemical but also of physical (energy) pollution. The concept of an energy threshold lies at the heart of ecological and hygienic standardization. If a physical impact exceeds the permissible level, irreversible changes occur in biological systems, reducing their viability. At the cellular and subcellular levels, metabolic, growth, and development processes react, and at the organism level, changes in behavior and general productivity are manifested.

Establishing maximum permissible levels (MPLs) of energy exposures is a complex task, as living organisms react physiologically in a non-uniform way to external fields. At present, there are no unambiguous, universal MPL values for all types of physical exposures. However, the established scientific methodology allows for the development of ecological standardization in strict accordance with the demands of agricultural practice.

Basic Concepts of Dosimetry and Radiation Exposure Limits

Safety monitoring of sewage sludge includes the assessment of radioactivity — the spontaneous decay of atomic nuclei with the transformation of chemical elements or their isotopes. Each act of decay is accompanied by the emission of alpha or beta particles, neutrons, or gamma quanta capable of ionizing the environment. Substances containing radionuclides (atoms with different numbers of nucleons, protons, or neutrons) are sources of ionizing radiation (IR). Radiation can be both external (from sources outside the organism) and internal (when radionuclides are ingested).

• Natural background radiation in the RF — 0.1…0.2 µSv/h
• Annual equivalent dose of natural background — 0.9 mSv
• 1 Becquerel (Bq) — 1 decay/s
• 1 Curie (Ci) — 3.7·10¹⁰ Bq
• 1 Gray (Gy) — 1 J/kg
• 1 Roentgen (R) — 2.58·10⁻⁴ C/kg

The measure of radioactivity is the activity of the radionuclide in the source (A). For practical calculations, specific (Bq/kg, Ci/kg), volumetric (Bq/l, Ci/l), and surface activity (Bq/m², Ci/m²) are used. The absorbed dose (D) determines the amount of radiation energy transferred to a mass of substance. To account for the biological effect of different types of radiation, the absorbed dose is multiplied by the weighting factor $W_R$, resulting in the equivalent dose ($H_{T,R} = D \times W_R$).

To assess the risk of long-term radiation effects, an effective dose is used, which accounts for the radiosensitivity of specific tissues and organs using the coefficient $W_T$:

• 0.12 — red bone marrow, colon, lungs, and stomach;
• 0.05 — bladder, breast, liver, esophagus;
• 0.01 — skin and bone surface cells.

Summing individual effective equivalent doses of a group of people gives the collective effective equivalent dose for assessing probabilistic (stochastic) effects. To protect against X-ray and gamma radiation, the exposure dose ($D_x$) and dose rate ($P$), measured in Gy/s, Sv/s, or R/s, are calculated.

Standardization of Radiation Safety when Using Sludge and Fertilizers

When applying sewage sludge, organic and fertilizer, the agronomist deals with a technologically altered natural radiation background. It arises due to the use of raw materials with an increased content of natural radionuclides and the burning of fossil fuels. An artificial (man-made) radiation background is formed by radionuclides from nuclear weapons testing, accidents, and emissions from the nuclear fuel cycle. The total dose received by people consists of these two backgrounds, as well as medical procedures.

If fields or waste processing facilities are located near industrial enterprises, it is important to consider the regulations for special zones:

• Sanitary Protection Zone (SPZ) — the territory around a radiation source where, during its standard operation, the radiation level may exceed the dose limit (DL). A restrictive regime is in effect here, and the facility's dosimetry service maintains constant monitoring.
• Monitoring Zone — the territory where the influence of radioactive discharges and emissions is possible, and the exposure of residents may reach the DL. Radiation monitoring in this zone is carried out by the radiological services of the sanitary and epidemiological service (SES).

Exposure levels are regulated by the base document — "Radiation Safety Standards (RSS-99)". The established dose limits do not include the dose from the natural background and medical examinations. All exposed persons are divided into personnel (Category A — those working directly with radiation sources; Category B — those within their sphere of influence) and the general public. At the same time, the dose limits for Category B personnel are equal to 1/4 of the values for Category A.

For all categories of exposed persons, there are three classes of standards:

• Basic dose limits (DL): boundary values provided in safety standards;
• Permissible levels of multifactorial exposure: values derived from DL for a single type of exposure or radionuclide (annual intake limits — AIL, permissible average annual volumetric activities — AVA, specific activities — SA);
• Control levels: indicators (doses, activities, flux densities) that are set at the enterprise itself, taking into account the achieved safety, to ensure that the actual impact remains guaranteed below the permissible level.