Aeration Pores

Pseudocyphellae are formed on the upper and lower surface of several foliose and fruticose lichens. In Pseudocyphellaria they form irregular warts. The openings are filled by a network of short cells (Fig. 89). In very old and abnormally large pseudocyphellae algal cells may be deposited between the loose hyphae. These pseudocyphellae resemble soralia but the similarity is misleading as no soredia are produced. 

The pseudocyphellae of the fruticose lichen Cornicularia divergens (Fig. 84) are either shallow depressions in the cortex or they form pores that penetrate to the medulla (Fig. 94). The pseudocyphellae on the upper side of Cetrelia cetrarioides show a similar development (Fig. 95). 

In some lichens, i.e., Parmelia exasperatula and Placopsis cribellans, the remnants of isidia serve as aerating organs. When the isidia break off, they leave on the thallus little warts which have a central opening filled by loosely interwoven hyphae. 

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Humidity. The rate of carbonation varies with humidity of concrete for two reasons. First, as already seen in Chapter 2, diffusion of carbon dioxide within concrete is facilitated through the aerated pores, but it is very slow through those filled with water (the diffusion of CO2 in water is four orders of magnitude slower than in air). 

Note that the soHds density used ia this equation should be the tme soHds, ie, skeletal, density, because the gas ia the pores is also compressed. For Group A soHds the aeration gas should also be added evenly along the standpipe. 

The pore space of a soil may contain either water or a gaseous atmosphere. Thus the aeration of a soil is directly related to the amount of pore space present and to the water content. Soils of fine texture due to a high clay content contain more closely packed particles and have less pore capacity for gaseous diffusion than an open-type soil such as sand. 

Porous articles for filtration, aeration, diffusion, etc. can also be made from materials having the same chemical inertness as those used for the low-porosity articles such as storage vessels. The pore size of these articles can be controlled to suit the operating conditions encountered. 

The activated sludge process for domestic wastewater treatment was introduced to the world in 1914.1 Since then, many studies have been conducted to improve the oxygen transfer efficiency. Among the aeration devices introduced have been a porous diffuser, a filter type diffuser, a mechanical aeration device, an orifice type diffuser and a fine-pore air diffuser. The aeration market is in a substantial state of flux in the USA today. Emphasis on high efficiency has led many intensive research programmes to aim at the evaluation of the design, operation and control processes to improve overall system performance. 

BalHca and Ryu [158] correlated reductions in cell yield in Datura stramonium suspensions with the increased Reynolds stresses associated with higher aeration rates in a 1.2-1 ALR. A more recent study [159] of C. roseus suspensions cultivated in a 1.5-1 bubble column showed that the increased bubble sizes associated with both larger sparger pores and higher aeration rates caused a reduction in system performance. Here, also, it was postulated that the effects were due to increased Reynolds shear stresses in the flow field. However, it was not possible to rule out gas-stripping effects. 

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

The application system, called the biodrain, is installed within the treatment area. The biodrain aerates the soil column and any standing water. This cerates an aerobic environment in the pore spaces of the soil. Other gas mixtures can also be introduced to the soil column, such as the air/ methane mixtures used in the biodegradation of chlorinated organics. The treatment platforms can be placed in very dense configurations. International Environmental Technology claims that the cost of installation is low. 

Shammas, N.K., Fine pore aeration of water and wastewater, in Advanced Physicochemical Treatment Technologies, Wang, L.K., Hung, Y.T., and Shammas, N.K., Eds., The Humana Press, Totowa, NJ, 2007, pp.391-448. 

Where the pores have only one opening or restricted openings, they do not drain and the interiors are reducing even when the soil in well aerated. The reducing conditions lead to the production of species such as methane and Fe2+. Although methane and iron are two of the most easily found and identified, other reduced species, both organic and inorganic, are commonly present. 

Soil Gas The minmum 02 concentration that can support aerobic metabolism in unsaturated soil is approximately 1%. 02 diffuses into soil because of pressure gradients, and CO 2 moves out of soil because of diffusivity gradients. Excess water restricts the movement of 02 into and through the soil. A minimum air-filled pore volume of 10% is considered adequate for aeration. Soil gas surveys using a mobile geoprobe unit have become a valuable tool to demonstrate a zone of enhanced microbial metabolism in the subsurface. 

The COj concentration in the subsurface may be different in small and large pores and vary as a function of the aerobic or anaerobic activity of the microbial population. Paul and Clark (1989) showed that a change from aerobic to anaerobic metabolism occurs at an concentration of less than 1% (by volume). The overall aeration of the soil layer is not as important as that of individual aggregates. Calculations show that water-saturated aggregates larger than 3 mm in radius have no in their center (Harris 1981). This means that aerobic and anaerobic zones may coexist in a porous medium even under partially saturated conditions. 

Enviroquip Flat-panel membranes with 0.4-pm pore size arranged vertically in aeration tanks. The membranes had the ability to relax. 

Zenon Microfiber membranes with 0.04-pm pore size. They were arranged in a vertical position in an aeration tank. The membranes were relaxing and back-pulsing. 

Huber Flat-panel membranes with 0.025-mm pore size. They are arranged in a vertical position on a rotating shaft in an aeration tank. They were subjected to spray washing. 

A typical well passes through two zones an upper zone, usually of soil-covered rock, that contains air and water in pores and fissures and a lower zone, often of rock, that contains only water in pores and fissures. This separation into two parts is observed everywhere, and the two zones have been named the aerated (air-containing) zone and the saturated (water-saturated) zone. The top of the saturated zone (where water is hit in a well) is called the water table (Fig. 2.1). The water table is described either in terms of its depth from the surface or as the altitude above sea level (section 2.2). 

Fig. 2.1 A schematic section through typical wells at the upper part the soil and rocks contain air and water in pores and fissures, forming the aerated zone. Below occur rocks with only water in their pores and fissures, forming the saturated zone. The top of the saturated zone is the water table, recognizable in wells as the depth at which water is encountered.

Intake of water in the aerated zone is either by infiltration into the soil cover or, on bare rock surfaces, by infiltration into intergranular pores (as in sand-stone), fissures and joints (as in igneous rocks or quartzite), or dissolution conduits and cavities (limestone, dolomite, gypsum, rock salt). Only pores and fissures that are interconnected, or communicate, are effective to infiltration. 

The mode of fluid flow in the aerated zone may be changed by the hydraulic aspects of fluid waste disposal. Constant release of large amounts of fluids may cause a local rise in the hydraulic head. Coupled with chemical fluid-rock interactions, this may form new high-conducting conduits that can lead the contaminants directly into the saturated zone. In other cases, fine particles that come with the contaminating fluids can clog pores in the aerated zone and reduce through-flow. 

Marx, J. and M. G. Rieth (1997). QA for effective fine pore aeration diffusers in wastewater treatment systems. ASCE Annu. Convention on Innovative Civil Eng. for Sustainable Development Quality Assurance—A National Commitment Proc. 1997 ASCE Annu. Convention on Innovative Civil Eng. for Sustainable Development, Oct. 5-8, Minneapolis, MN, 132-141. Sponsored by ASCE, New York. 

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