Capítulo I. El mercado financiero formal
1.5 Tipos de mercados financieros formales
1.5.4 Mercado de divisas o Forex
Facultative ponds are classified into two types; primary facultative ponds that receive raw wastewater (after pre-treatment units such as screening and grit removal), and secondary facultative ponds that receive effluent of a primary treatment stage (usually effluent form anaerobic ponds). They are 1-2 m deep with 1.50 m is most commonly used. These ponds are designed for the removal of BOD based on ‘surface BOD loading’ rates which refer to the amount of organic matter (Kg BOD) applied each day for each hectare of the surface area of the pond and thus the resulting overall unit is (Kg BOD /ha. day). Relatively low surface BOD loading rates ranging from 100 to 400 Kg BOD /ha. day are used in designing the facultative ponds to maximise the surface area of the pond exposed to the sunlight and thus
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allowing for the growth of healthy algal population which by their photosynthetic activity produce most of the oxygen required by heterotrophic bacteria to consume the organic matter (Mara, 1992; Mara, 2003; Mara and Peňa, 2004). Due to the profuse growth of algae, facultative ponds are of dark green colour. The most predominant algae in facultative ponds are of motile genera (such as Pryobotrys and Chlamydomonas) which can more easily optimize their position according to incident light intensity than non motile algae (Mara, 1992; Mara, 2003). Facultative ponds may occasionally turn into red or pink (particularly when overloaded) due to the huge growth of anaerobic purple sulphide – oxidising photosynthetic bacteria at such conditions. The algal concentration in facultative ponds mainly depends on the temperature and loading, and they are best expressed in term of their main photosynthetic pigment (i.e. chlorophyll a per litre). In a well –performing facultative pond, algal biomass ranges from 500-2000 chlorophyll a /l (Mara, 1992; Mara, 2003; Mara and Peňa, 2004).
There is a mutualistic relationship between bacteria and algae in facultative ponds (Figure 3.3); oxygen required by the bacteria to oxidise the organic matter is provided by algae, and the carbon dioxide required by the algae is provided by bacteria. Photosynthetic activities of algae result in a diurnal variation of the in-pond dissolved oxygen (DO) levels as well as pH (Mara, 1992; Mara, 2003; Mara and Peňa, 2004). In-pond dissolved oxygen concentration gradually rises after sunrise to a maximum level at the mid afternoon which then falls till a minimum at the night when photosynthetic activities of algae cease (Figure 3.4). In-pond pH similarly changes and peaks at peak algal activity; when algae are photosynthesizing rapidly, their demand of carbon dioxide (
co
) exceeds that supplied by bacteria and thus causing bicarbonate and carbonate ions to dissociate and to form moreco
for algae while resulting hydroxyl ions (OH) accumulate and thereby pH values rises, in most cases, to more than 9.40 (Mara, 1992; Mara, 2003; Mara et al. 2003). The following chemical formulas represent the formation process of (co
2)
and (OH). Both high levels of dissolved oxygen and pH are essential for bacterial die-off (Mara, 1992; Mara, 2003; Mara et al. 2003; Mara and Peňa, 2004).1) 2HCO → CO + HO + CO
Bicarbonate → Carbonate + Water + Carbon dioxide
2) CO + HO → CO + 2OH
Carbonate + water → carbon dioxide + Hydroxyl ions
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The behaviour of facultative ponds is highly affected by wind that induces vertical mixing within the pond content. Good mixing results in a more uniform distribution of algae, bacteria, BOD and DO and thus allowing for better stabilisation of the wastes. In the absence of the wind, algal stratification took place where algae stratifies in a 20 cm band and moves vertically within the top 50 cm of the pond with respect to the intensity light intensity. Such stratification may cause the effluent quality (BOD and SS) to largely fluctuate if the effluent take off point is within the stratification zone (Mara, 1992; Mara, 2003).
Figure 3. 3 Mutualistic relationship between bacteria and algae in facultative and maturation ponds.
Source: Mara and Peňa (2004)
Figure 3. 4 Diurnal variations of in-pond DO in a facultative ponds (top 200 mm of the pond; curve of open circles) and (800 mm below the surface; curve of solid circles). Source: Mara (2003)
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39 Design of facultative ponds
Facultative ponds are designed based on surface BOD loading (*S , kg BOD / ha.day), which is given by;
*
S=
uvw (3.6)Where, T is the BOD on the effluent of anaerobic pond for secondary facultative ponds or the BOD on the raw wastewater for primary facultative ponds, expressed in mg/l or g/m .
Q is the wastewater flow entering the facultative pond, m /day. Note: TQ is the BOD mass entering the facultative ponds, g /day
l is the facultative pond area, mThe factor 10 is used to convert the input units to that of *S; 10 TQ (Kg /day) / 108 l
The allowable design value of *S is function of temperature (T, ℃ ) as it increases with it.
Mara (1987) developed the global design equation for calculating *S values; this equation is based on the experience on the facultative ponds behaviour at different surface loading rates and temperatures:
*S = 350(1.107 − 0.002z) (3.7) z is the design temperature (the mean temperature of the coldest month, ℃) . Once T is known and *S is calculated, the facultative pond area then can be calculated from equation 3.6. The hydraulic retention time of the pond can then be calculated from;
@
l=
wK(3.8)
Where, D is the facultative pond depth (1-2 m), with 1.5 m most commonly used. Depth of less than 1m is not recommended to avoid mosquitoes breeding problems as this encourages the growing of rooted plants that provide ideal shaded habitats for mosquitoes. Depths larger than 1.8m also are not recommended to avoid anaerobic conditions to predominate as the oxypause in such cases is near the pond surface (Mara, 2003). However, Mara (2003) recommends depths of larger than 2 m to minimize the pond surface area and thus reducing the water losses result from high evaporation rates.
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m is the mean flow of the influent ( T ) and the effluent flow ( ). The latter is the influent flow minus net evaporation and seepage. Equation 2.34 then becomes;
@
l=
[.(wvKw)](3.9)
By neglecting the seepage, can be given from;
= T− 0.001 l (3.10) Where, is the net evaporation rate (measured in mm/day) which can be obtained from metrological stations records
By substituting in equation 3.9 by its expression in equation 3.10, equation 3.9 thus becomes;
@
l=
[wv.K ](3.11)
The hydraulic retention time @l of facultative pond should not be less than 5 days for T < 20
℃ and not less than 4 days for T > 20 ℃ to avoid short circuiting (Mara, 2003). Thus, if @l calculated from equation 3.11 is less than the permissible minimum value, the minimum value of @l should be used and l should be recalculated.
The BOD in the effluent of facultative ponds () can be estimated from the from the following equation;
=
uvV(3.12)
Where, T is the BOD (mg/l) of either the effluent of anaerobic pond in case of secondary facultative pond, or the raw wastewater in case of primary facultative ponds.
is the first order rate constant for BOD removal, day
is a temperature related parameter and is given by the following equation (Arrhenius equation);
() = ()(1.05) (3.13)
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Where, () is the value of at T ℃ and () is the value of at 20 ℃. Mara (2003) recommends design values for () for primary and secondary facultative which are; 0.30 and 0.10 day, respectively. () of secondary facultative ponds is much lower than that of primary types as most of the BOD removal is achieved by sedimentation in the anaerobic pond preceding secondary facultative ponds.
The term , calculated from equation 3.12, is unfiltered BOD that includes algal BOD. This algal BOD ranges from ~ 70 to 90 percent of the total BOD in the effluent of a facultative pond (unfiltered effluent). Thus, filtered BOD (non-algal BOD) can be given by the following equation (Mara, 2003);
(iltered) = U × [(unfiltered)] (3.14) Where, U is the non algal fraction of the total BOD ~ 0.1 to 0.3, with a suitable design value of 0.30 (Mara, 2003).
Ponds effluent with filtered BOD not larger than 25 mg/l is considered by the European Union regulation as an acceptable effluent to be discharged in surface water (Mara, 2003).
Mara (2003) also urges regulators in developing countries to consider this acceptable level.
Microbiological quality assessment: It is important at this stage of the WSP design to assess the microbiological effluent quality and its suitability for reuse purpose (i.e. crop irrigation).
In other words, number of human intestinal nematode eggs and E.coli in the effluent.
The number of nematode eggs in the effluent can be calculated from the following equation;
No. of eggs in the efluent = No of eggs in inluent (1 − R) (3.15) Where, R is the percentage of egg removal achieved by each pond. Ayres et al. (1992), using data of WSP performance in different countries, developed an equation for R which correlates it with the retention time; they also showed that this equation can be used for all the different types of WSP. R thus can be calculated from;
R = 100[1 − 0.41 exp(−0.49θ + .0085 θ)] (3.16) For a series of ponds, for example at this stage, comprising anaerobic and secondary facultative pond, the number of eggs in the final effluent can be calculated from the following equation;
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No. of eggs in the efluent = No of eggs in raw wastewater P1 − R>P1 − R> (3.17) The World Health Organization (WHO) (2004) sets maximum permitted number of nematode eggs in the effluent that to be reused either for restricted irrigation (irrigation of all crops except vegetables and crops eaten uncooked); or for unrestricted irrigation (including crops eaten uncooked). Compliance with WHO requires that the effluent should contain less than 1 egg per litre of the effluent and less than 0.10 egg if children under the age of 15 are exposed to irrigation field for both restricted and unrestricted irrigation. Thus, if the calculated number of eggs in the effluent is more than these limits, maturation ponds should be included to produce effluent that fit the purpose (Mara, 2003).
In term of E.coli numbers, the number of E.coli can be determined as described in the following section using equation 3.18, 3.19 and 3.20. The World health organisation (2004) also stipulates that effluent should not contain more than not more than 10 E.coli per 100 ml of the effluent for restricted and 1000 E.coli per 100 ml for unrestricted irrigation.