Aeration Regimes Research Articles

ФІТОІНДИКАЦІЙНА ОЦІНКА ДИНАМІКИ РЕЖИМУ АЕРАЦІЇ ТЕХНОЗЕМІВ НІКОПОЛЬСЬКОГО МАРГАНЦЕВОРУДНОГО БАСЕЙНУ

In the work the regularities of the regime of technosols aeration dynamics in Nikopol manganese ore basin after agricultural reclamation were revealed. Experiments were conducted at the research station of Dnipro State Agro-Economic University (Pokrov, Dnipropetrovsk region, Ukraine). The temporal dynamics of phytoindication indexes changes was approximated by a Hill's equation. The analytical dependence between parameters of plant aeromorphes and soil porosity was proposed. It was shown that in the progress of the technosols soil forming process the increase of soil aeration regime is an important trend that in the vast majority of undermining the ecological conditions in them further from optimal for the vast majority of cultivated plants. In addition, this tendency makes the extreme conditions of existence for natural species of plants and pedobiont animals. A tendency to increase aeration can be seen as a stage in a longer process, which must be stabilized and lead to the formation of ecological modes of technosols, close to the natural analogues. Keywords : air mode, aeromorphes, phytoindication, reclamation, temporal dynamics, succession

Aeration of different irrigation levels affects net global warming potential and carbon footprint for greenhouse tomato systems

Abstract The study of aerated irrigation (AI) in conjunction with different irrigation levels on economic and environmental effects has important scientific significance for the selection of optimal irrigation and reasonable greenhouse management practices. In this paper, a two-year experiment was conducted to investigate tomato yield, water use efficiency (WUE), CO2, N2O, CH4 emissions, net global warming potential (NGWP), and carbon footprint (CF) differences for a greenhouse tomato system in Northwestern China in 2016 and 2017. Based on the amount of irrigation needed to provide an adequate water supply (W), 60%W, 80%W and 100%W were set as the three irrigation levels, with two aeration regimes (aeration and control), totaling six treatments. Compared to the control, AI significantly increased tomato yield by 32.0%, WUE by 32.0% and CF by 24.0% on average (p 0.05). With respect to the treatment of 100%W, 80%W over the two years had no obvious effect on tomato yield, WUE and CF (p>0.05), while 60%W reduced yield by 23.4% and CF by 32.6% but increased WUE by 35.1% significantly (p 0.05). The main inputs in this arid and semi-arid region were electricity for irrigation and fertilizers, contributing 87.9%–92.9% of the total impact. Overall, the treatment of aerated full irrigation was suitable for crop production, water saving and carbon sequestration with WUE of 24.05 (in 2016) and 19.95 kg m−3 (in 2017), a net greenhouse gas intensity of 0.103 (in 2016) and 0.086 (in 2017) t t−1, and with a carbon footprint per tomato yield of 0.350 (in 2016) and 0.426 (in 2017) kg CO2-eq kg−1.

CFD simulation of fluid dynamic and biokinetic processes within activated sludge reactors under intermittent aeration regime

Due to the aeration system, biological reactors are the most energy-consuming facilities of convectional WWTPs. Many biological reactors work under intermittent aeration regime; the optimization of the aeration process (air diffuser layout, air flow rate per diffuser, aeration length …) is necessary to ensure an efficient performance; satisfying the effluent requirements with the minimum energy consumption.This work develops a CFD modelling of an activated sludge reactor (ASR) which works under intermittent aeration regime. The model considers the fluid dynamic and biological processes within the ASR. The biological simulation, which is transient, takes into account the intermittent aeration regime. The CFD modelling is employed for the selection of the aeration system of an ASR. Two different aeration configurations are simulated. The model evaluates the aeration power consumption necessary to satisfy the effluent requirements. An improvement of 2.8% in terms of energy consumption is achieved by modifying the air diffuser layout. An analysis of the influence of the air flow rate per diffuser on the ASR performance is carried out. The results show a reduction of 14.5% in the energy consumption of the aeration system when the air flow rate per diffuser is reduced. The model provides an insight into the aeration inefficiencies produced within ASRs.

TECHNOLOGICAL REGIMES FOR OBTAINING VINEGAR FROM BEER DIALYSTS

This article is devoted to the development of technological regimes for the rational use of beer dialysates formed in the production of non-alcoholic beer. One of the effective ways to use it, is the production of vinegar. The article presents data on the study of volatile components, organic acids and amino acids of initial beer dialysates with a volume fraction of ethyl alcohol of 0.6 % and brewing dialysates concentrated to a volume fraction of ethyl alcohol of 5.0 % and 8.0 %. The data of the study of volatile components, organic acids and amino acids of vinegar obtained as a result of biochemical oxidation of concentrated brewing dialysates with acetic acid bacteria are presented. The oxidation process was carried out by a periodic deep-seated method. Influence of aeration regimes and initial concentration of acetic acid on the functional activity of acetic acid bacteria in obtaining vinegar from beer dialysates is shown. Recommended technological regimes for obtaining vinegar in a periodic deep method from beer dialysates.

ВІТРОВИЙ РЕЖИМ ВЕЛИКОГО МІСТА

Big city wind regime is an important factor which influences on air pollution level and bioclimate of the territory. It forms as a result of climatic conditions influence and the impact of various objects (height and density of construction, road network, green zones, water bodies etc.) within the city. The article is devoted to the study and analysis of big city wind regime, as well as generalizing of the basic knowledge about air flow transformation in urban areas for future using this information in scientific research and for the correction of aeration regime of different territories. Wind speed reduction is the main feature of urban areas wind regime. Usually, wind velocity in the city is approximately 30 % lower compared to its neighbourhoods. Not only the mean wind speed is lower in urban areas, but also the vertical wind profile is characterized by some differences. Urban heat island existence leads to the formation of a mesoscale thermal circulation in the city – a rural breeze which is the most pronounced under conditions without wind or with weak wind speed. Transformation of air flow in an urban street canyon depends on the wind direction – when the wind is directed parallel to the canyon axis, wind within canyon has a higher velocity (according to the Venturi effect). In a street canyon which is oriented with its axis perpendicular to the wind direction, when air flows across the cross-section of a canyon, a vortex is formed and accordingly wind velocity is getting lower. Depending on the characteristics of the canyon (its height/width and length/width ratio) three distinct wind regimes may be identified in the canyon – isolated roughness flow regime, wake interference regime and skimming flow regime. Thus, the wind field has a very complex structure in the urban areas, because on airflow in the city influence a lot of factors.

Empirical operating strategy to reduce the light-specific energy consumption of a flat-panel airlift photobioreactor with intrinsic static mixers cultivating Thermosynechococcus elongatus BP-1

During photoautotrophic cultivations using optimized synthetic media, light is the limiting “substrate” of high-density cultures, especially throughout outdoor cultivations prohibiting adjustment of photon-flux density (PFD). Convective mass transfer (turbulence) is the method of choice to cope with that limiting effect. Then again, turbulence comes at costs through the energy required for its generation. In this context and based on laboratory data generated with Thermosynechococcus elongatus BP-1, an empiric operating strategy for flat-panel airlift photobioreactors with intrinsic static mixers is suggested. Repeated cultivations were performed from sub- to supra-saturating PFD (180 to 780μmolm−2s−1) at aeration rates ranging from 0.11 to 0.83vvm assessing the cultures' productivities along the courses of biomass concentrations. The results indicate that, owing to the culture-flow directing mixers, there is a strong interrelation between the effects of PFD, biomass concentration and aeration rate (thus energy input) applied. Hereby, the positive impact of directed turbulence on the cultures' performance with respect to productivity and yield was greatest at high biomass concentration (>5gDWL−1) and PFDs with effects becoming less dominant while the latter two were reduced. Based on the quantitative findings and utilizing the easily monitored parameters PFD and biomass concentration, a computational model may be defined automatically controlling the easily adjustable parameter aeration rate. Without a negative influence on cultures' performance, this will allow for both, increase of culture performance at times of intense light and high biomass concentration as well as reduction of operational expenditures (OPEX) at times of dim light or low biomass concentration. Compared to the standard, continuous aeration regime, this may lead to a reduction of energy required for the generation of turbulence of 37%, especially when considering outdoor cultivations in temperate climate zones.

Nitrous oxide production in intermittently aerated Partial Nitritation-Anammox reactor: oxic N2O production dominates and relates with ammonia removal rate

Emissions of the greenhouse gas nitrous oxide from the Partial Nitritation-Anammox process are of concern and can determine the carbon footprint of the process. In order to reduce nitrous oxide emissions intermittent aeration regimes have been shown to be a promising mode of operation, possibly due to an effective control of accumulation of nitrogen intermediates. However, due to frequent changes of redox conditions under intermittent aeration regimes, nitrous oxide production and emissions are dynamic. In this study the production and emission dynamics of nitrous oxide in an intermittently aerated sequencing batch reactor were monitored in high temporal resolution, the contribution of different redox conditions to overall nitrous oxide production was quantified and the most relevant factors for nitrous oxide production were identified. The average fraction of nitrous oxide produced (per unit ammonium removed) was 1.1 ± 0.5%. Cycle-averaged approx. 80% of nitrous oxide was produced during aerated phases, the remaining 20% were produced during non-aerated phases. Yet, the intra-cycle dynamics of nitrous oxide were substantial. The net-production rate of nitrous oxide during aerated phases correlated with the ammonia removal rate, whereas the concentration of nitrite determined the production during non-aerated phases. While aerated phases contributed predominantly at the beginning of reactor cycles, non-aerated phases became the dominant source of nitrous oxide at the end. Particularly low net-production rates were observed at ammonia removal rates below 5 mg NH3-N*gVSS−1*L−1, when the fraction of nitrous oxide produced was 0.011 ± 0.004% (per ammonia removed). Based on the nitrous oxide dynamics and correlations, reactor operation at relatively low nitrogen loadings (below 100 mg NH4+-N*L−1), ammonia removal rates of approx. 5 mg NH3-N*gVSS−1*L−1 and nitrite concentrations below 1 mg NO2−1-N*L−1 appears as beneficial for low emission of nitrous oxide.

Systems Biology Approach to Bioremediation of Nitroaromatics: Constraint-Based Analysis of 2,4,6-Trinitrotoluene Biotransformation by Escherichia coli.

Microbial remediation of nitroaromatic compounds (NACs) is a promising environmentally friendly and cost-effective approach to the removal of these life-threating agents. Escherichia coli (E. coli) has shown remarkable capability for the biotransformation of 2,4,6-trinitro-toluene (TNT). Efforts to develop E. coli as an efficient TNT degrading biocatalyst will benefit from holistic flux-level description of interactions between multiple TNT transforming pathways operating in the strain. To gain such an insight, we extended the genome-scale constraint-based model of E. coli to account for a curated version of major TNT transformation pathways known or evidently hypothesized to be active in E. coli in present of TNT. Using constraint-based analysis (CBA) methods, we then performed several series of in silico experiments to elucidate the contribution of these pathways individually or in combination to the E. coli TNT transformation capacity. Results of our analyses were validated by replicating several experimentally observed TNT degradation phenotypes in E. coli cultures. We further used the extended model to explore the influence of process parameters, including aeration regime, TNT concentration, cell density, and carbon source on TNT degradation efficiency. We also conducted an in silico metabolic engineering study to design a series of E. coli mutants capable of degrading TNT at higher yield compared with the wild-type strain. Our study, therefore, extends the application of CBA to bioremediation of nitroaromatics and demonstrates the usefulness of this approach to inform bioremediation research.

Model-based biotechnological potential analysis of Kluyveromyces marxianus central metabolism.

The non-conventional yeast Kluyveromyces marxianus is an emerging industrial producer for many biotechnological processes. Here, we show the application of a biomass-linked stoichiometric model of central metabolism that is experimentally validated, and mass and charge balanced for assessing the carbon conversion efficiency of wild type and modified K. marxianus. Pairs of substrates (lactose, glucose, inulin, xylose) and products (ethanol, acetate, lactate, glycerol, ethyl acetate, succinate, glutamate, phenylethanol and phenylalanine) are examined by various modelling and optimisation methods. Our model reveals the organism's potential for industrial application and metabolic engineering. Modelling results imply that the aeration regime can be used as a tool to optimise product yield and flux distribution in K. marxianus. Also rebalancing NADH and NADPH utilisation can be used to improve the efficiency of substrate conversion. Xylose is identified as a biotechnologically promising substrate for K. marxianus.

Micropropagation of axillary shoots of hybrid chestnut (Castanea sativa × C. crenata) in liquid medium in a continuous immersion system

A protocol for culturing chestnut axillary shoots with the basal sections continuously immersed in liquid medium was developed and the influence of various parameters was assessed: explant type and size, support material, bioreactor size and aeration regime. Shoots excised from eight selected chestnut genotypes were successfully cultured in 10 L bioreactors in liquid Murashige and Skoog medium with half strength nitrates and supplemented with 0.05 mg/L N 6-benzyladenine and 30 g/L sucrose. Forced ventilation and the use of a support material were essential for producing healthy, non-hyperhydric shoots. High proliferation rates were achieved and good quality shoots ready for multiplication, rooting and acclimatization were obtained. Larger leaves showing higher levels of photosynthetic pigments were observed in shoots cultured in bioreactors, suggesting a certain degree of photoautotrophy in shoots cultured under forced ventilation. This is the first demonstration of the production of chestnut shoots in stationary liquid medium, indicating the feasibility of the method for large-scale propagation of the species.

Biodrying performance and bacterial community structure under variable and constant aeration regimes during sewage sludge biodrying

ABSTRACTThis study investigated biodrying performance under variable and constant aerations. The matrix-related properties, air-related parameters, and bacterial community were assessed. Under variable aeration, the velocity and temperature of the air above the pile surface were high; bacterial diversity indicated that the biodrying was achieved during an established period; Proteobacteria was dominant throughout the biodrying; and there was better performance of the biodrying product intended for direct incineration or for postcuring land use. Different aeration conditions led different biomass percentages, but the dominant phyla were similar.

粉体のレオロジー評価と流動性に関する考察

This study evaluates how powders are affected by air using Powder Rheometer which can measure the torque for powder viscosity and flow energy. It can be challenging to directly evaluate the properties of an aerated powder, as glass beads and MCC only require a small amount of air to change their behavior, but flour may require significantly more, or will be insensitive to aeration. The aeration characteristics of a powder can be studied with powder rheology by measuring their flow energy during aeration. This evaluation demonstrates how it is possible to evaluate the flow characteristics of powder using different aeration regimes.

Enzymatic biofilm digestion in soil aggregates facilitates the release of particulate organic matter by sonication

Abstract. The stability of soil aggregates against shearing and compressive forces as well as water-caused dispersion is an integral marker of soil quality. High stability results in less compaction and erosion and has been linked to enhanced water retention, dynamic water transport and aeration regimes, increased rooting depth, and protection of soil organic matter (SOM) against microbial degradation. In turn, particulate organic matter is supposed to support soil aggregate stabilization. For decades the importance of biofilm extracellular polymeric substances (EPSs) regarding particulate organic matter (POM) occlusion and aggregate stability has been canonical because of its distribution, geometric structure and ability to link primary particles. However, experimental proof is still missing. This lack is mainly due to methodological reasons. Thus, the objective of this work is to develop a method of enzymatic biofilm detachment for studying the effects of EPSs on POM occlusion. The method combines an enzymatic pre-treatment with different activities of α-glucosidase, β-galactosidase, DNAse and lipase with a subsequent sequential ultrasonic treatment for disaggregation and density fractionation of soils. POM releases of treated samples were compared to an enzyme-free control. To test the efficacy of biofilm detachment the ratio of bacterial DNA from suspended cells and the remaining biofilm after enzymatic treatment were measured by quantitative real-time PCR. Although the enzyme treatment was not sufficient for total biofilm removal, our results indicate that EPSs may attach POM within soil aggregates. The tendency to additional POM release with increased application of enzymes was attributed to a slight loss in aggregate stability. This suggests that an effect of agricultural practices on soil microbial populations could influence POM occlusion/aggregate stability and thereby carbon cycle/soil quality.

Sludge reduction using aquatic worms under different aeration regimes

ABSTRACTAdding aquatic worms to a wastewater treatment system can reduce sludge production through predation. The aeration level is crucial for success. To evaluate aeration impacts on sludge reduction and determine an optimal aeration regime, this study investigated the processes of in-situ sludge reduction, using aquatic worms exposed to different aeration levels. The experiment also compared treatment results between a conventional reactor and an aquatic worm reactor (WR). Results indicated that the recommended concentration of dissolved oxygen (DO) was 2.5 mg L−1. The removal rate of chemical oxygen demand remained steady at 80% when the DO concentration was higher than 2.5 mg L−1, while the removal rate of ammonia nitrogen continued to moderately increase. Increasing the DO concentration to 5 mg L−1 did not improve sludge reduction, and consumed more power. With a DO concentration of 2.5 mg L−1 and a power of 0.19 kWh t−1 water, the absolute sludge reduction and relative sludge reduction rates in the WR were 60.0% and 45.7%, respectively, and the daily aquatic worm growth rate was 0.150 d−1 during the 17-d test. Therefore, at the recommended aeration regime, aquatic worms reduced the sludge without increasing the power consumption or deteriorating the effluent.

Hydrolytic anaerobic reactor and aerated constructed wetland systems for municipal wastewater treatment – HIGHWET project

ABSTRACTThe HIGHWET project combines the hydrolytic up-flow sludge bed (HUSB) anaerobic digester and constructed wetlands (CWs) with forced aeration for decreasing the footprint and improving effluent quality. The HIGHWET plant in A Coruña (NW of Spain) treating municipal wastewater consists of a HUSB and four parallel subsurface horizontal flow (HF) CWs. HF1, HF2 and HF3 units are fitted with forced aeration, while the control HF4 is not aerated. All the HF units are provided with effluent recirculation, but different heights of gravel bed (0.8 m in HF1 and HF2, and 0.5 m in HF3 and HF4) are implemented. Besides, a tobermorite-enriched material was added in the HF2 unit in order to improve phosphorus removal. The HUSB 76–89% of total suspended solids (TSS) and about 40% of chemical oxygen demand (COD) and biological oxygen demand (BOD). Aerated HF units reached above 96% of TSS, COD and BOD at a surface loading rate of 29–47 g BOD5/m2·d. An aeration regime ranging from 5 h on/3 h off to 3 h on/5 h off was found to be adequate to optimize nitrogen removal, which ranged from 53% to 81%. Average removal rates of 3.4 ± 0.4 g total nitrogen (TN)/m2·d and 12.8 ± 3.7 g TN/m3·d were found in the aerated units, being 5.5 and 4.1 times higher than those of the non-aerated system. The tobermorite-enriched HF2 unit showed a distinct higher phosphate (60–67%) and total phosphorus (54%) removal.

Absence of the cbb3 Terminal Oxidase Reveals an Active Oxygen-Dependent Cyclase Involved in Bacteriochlorophyll Biosynthesis in Rhodobacter sphaeroides.

ABSTRACTThe characteristic green color associated with chlorophyll pigments results from the formation of an isocyclic fifth ring on the tetrapyrrole macrocycle during the biosynthesis of these important molecules. This reaction is catalyzed by two unrelated cyclase enzymes employing different chemistries. Oxygenic phototrophs such as plants and cyanobacteria utilize an oxygen-dependent enzyme, the major component of which is a diiron protein named AcsF, while BchE, an oxygen-sensitive [4Fe-4S] cluster protein, dominates in phototrophs inhabiting anoxic environments, such as the purple phototrophic bacterium Rhodobacter sphaeroides. We identify a potential acsF in this organism and assay for activity of the encoded protein in a strain lacking bchE under various aeration regimes. Initially, cells lacking bchE did not demonstrate AcsF activity under any condition tested. However, on removal of a gene encoding a subunit of the cbb3-type respiratory terminal oxidase, cells cultured under regimes ranging from oxic to micro-oxic exhibited cyclase activity, confirming the activity of the oxygen-dependent enzyme in this model organism. Potential reasons for the utilization of an oxygen-dependent enzyme in anoxygenic phototrophs are discussed.IMPORTANCE The formation of the E ring of bacteriochlorophyll pigments is the least well characterized step in their biosynthesis, remaining enigmatic for over 60 years. Two unrelated enzymes catalyze this cyclization step; O2-dependent and O2-independent forms dominate in oxygenic and anoxygenic phototrophs, respectively. We uncover the activity of an O2-dependent enzyme in the anoxygenic purple phototrophic bacterium Rhodobacter sphaeroides, initially by inactivation of the high-affinity terminal respiratory oxidase, cytochrome cbb3. We propose that the O2-dependent form allows for the biosynthesis of a low level of bacteriochlorophyll under oxic conditions, so that a rapid initiation of photosynthetic processes is possible for this bacterium upon a reduction of oxygen tension.

Impact of carbon to nitrogen ratio and aeration regime on mainstream deammonification.

While deammonification of high-strength wastewater in the sludge line of sewage treatment plants has become well established, the potential cost savings spur the development of this technology for mainstream applications. This study aimed at identifying the effect of aeration and organic carbon on the deammonification process. Two 10 L sequencing bath reactors with different aeration frequencies were operated at 25°C. Real wastewater effluents from chemically enhanced primary treatment and high-rate activated sludge process were fed into the reactors with biodegradable chemical oxygen demand/nitrogen (bCOD/N) of 2.0 and 0.6, respectively. It was found that shorter aerobic solids retention time (SRT) and higher aeration frequency gave more advantages for aerobic ammonium-oxidizing bacteria (AerAOB) than nitrite oxidizing bacteria (NOB) in the system. From the kinetics study, it is shown that the affinity for oxygen is higher for NOB than for AerAOB, and higher dissolved oxygen set-point could decrease the affinity of both AerAOB and NOB communities. After 514 days of operation, it was concluded that lower organic carbon levels enhanced the activity of anoxic ammonium-oxidizing bacteria (AnAOB) over denitrifiers. As a result, the contribution of AnAOB to nitrogen removal increased from 40 to 70%. Overall, a reasonably good total removal efficiency of 66% was reached under a low bCOD/N ratio of 2.0 after adaptation.

Impact of aeration shear stress on permeate flux and fouling layer properties in a low pressure membrane bioreactor for the treatment of grey water

Abstract Two different aeration regimes were studied in a low pressure gravity driven membrane bioreactor without any flushing or (back-) washing. In one reactor, the aeration was positioned below the membrane module, thus exposing the membranes to aeration shear stress. A second reactor was operated at low shear stress by placing the aerator in a different compartment. Flux stabilization at 2.0 L/(m 2 h) occurred in the reactor with low shear stress while no flux stabilization was observed in the reactor with aeration shear stress, resulting in a flux of 0.5 L/(m 2 h) after 120 days. The thickness of the bio-fouling layer in the reactor with aeration shear was smaller (129 vs. 344 µm), which implies that shear stress resulted in a thinner, denser and less permeable bio-fouling layer. The results can be explained by differences in (1) the morphology of the bio-fouling layer and (2) the EPS contents (proteins and polysaccharides) in the bio-fouling layer. The low-shear system provides a suitable solution for decentralized grey water treatment, or other conditions where maintenance and energy consumption should be minimized. Furthermore, the results can contribute to decrease the energy consumption in MBR systems.

<b>Interaction between afternoon aeration and tilapia stocking density

The present study aimed at determining the effects of the interaction between afternoon aeration and stocking density of Nile tilapia on variables of water and soil quality, growth performance and effluent quality. The experiment was a 3 x 2 factorial randomized block design, with three stocking densities (8, 12 and 16 fish per tank or 43.5, 65.3, and 87.0 g m-3) under two mechanical aeration regimes, absence (control; three replicates) and afternoon aeration (four replicates). The afternoon aeration was carried out from 12.00 a.m. up to 18.00 p.m. from the 3rd week until the end of the experiment. Except for the 16-fish tanks, the lowest concentrations of total ammonia nitrogen were found in the tanks with higher density of fish provided with afternoon aeration. Nitrite concentrations were lower in the 8-fish aerated tanks. In intensive system, the afternoon aeration of the fish culture water is an efficient management of water quality to remove gaseous ammonia and nitrite from water, but it is not appropriate to remove hydrogen sulfide from water.

The importance of aeration mode and flowrate in the determination of the biological activity and stability of organic wastes by respiration indices

The aim of this study was to assess the effect of different air flowrates and different aeration modes on the respiration activity of three organic substrates of different stability degree: (i) a constant flowrate and (ii) a continuously adjusted air flowrate that optimized the oxygen uptake rate (OUR). Above 20L air kg−1DMh−1, at the constant flow regime, the resulting dynamic respiration index at 24h (DRI24) and the cumulative respiration at four days (AT4) were statistically similar. At the OUR based aeration regime, the DRI24 and AT4 were statistically similar at all initial flowrates tested. Above a minimum threshold, cumulative air flow of around 3000Lairkg−1 DM during a 5day period, the respiration activity was similar, particularly for the two less active substrates. This study highlights the importance of selecting the aeration to obtain reliable measures of biological activity and stability in organic wastes.