Pilar 3. Construcción de Comunidad
9. Formato de Entrevista Semi Estructurada
Maturation ponds are the last ponds in a series of ponds of a WSP system and they receive effluent from facultative ponds. Their primary function is the reduction of excreted pathogens, mainly faecal viruses and bacteria, to acceptable levels or levels suitable for reuse purposes (i.e. aquacultural or agricultural purposes), and this is extremely effective in a well designed, operated and maintained series of ponds in a warm climate (see Appendix 3.1 ).
However, some BOD is removed in maturation ponds and they have significant contribution to the removal of nutrients (nitrogen and phosphorus) (see also Appendix 3.1). The number and size of maturation ponds is mainly governed by the desired microbiological quality of the final effluent (Mara, 1992; Mara, 2003; Mara and Peňa, 2004).
Maturation pond depth ranges from 1- 1.5 m, with 1 m is most commonly used to allow for effective light penetration which is essential in the die-off of faecal bacteria and viruses , but depth of less than 1 m is not recommended due to associated problems of mosquitoes breeding as illustrated former in facultative ponds). Because of their shallower depths and lower organic loading rates, they are typically aerobic or well oxygenated (Mara, 1992; Mara, 2003; Mara and Peňa, 2004).
Maturation ponds show less physiochemical and biological stratification than facultative ponds. They also have lower algal biomass but much more diverse than that in facultative
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ponds with non motile algae tends to predominate in maturation ponds. The algal diversity increases from pond to another along a series of maturation ponds, while the algal biomass decreases (Mara, 1992; Mara, 2003).
Pathogen removal mechanism
Faecal viruses: are mainly removed by adsorption on settleable solids followed by sedimentation of these solids. This also includes adsorption on algae in maturation and facultative ponds followed by sedimentation of algae as they die off. (Mara, 1992; Mara;
2003, Mara et al. 2003)
Faecal bacteria: Mara (2003) divides faecal bacteria removal processes into two major groups; ‘dark–mediated’ processes and ‘light mediated processes’. Dark – mediated processes refers to light independent processes that occur in both the dark and the light. These processes include adsorption of faecal bacteria on to seatleable solids which is then followed by sedimentation of these solids; predation by free - living organisms such as protozoa; and die-off due senescence and starvation.
In the other hand, light- mediated processes are light related process and they are highly influenced by the following factors (Mara, 1992; Mara, 2003):
Time and temperature: the higher the pond temperature and the longer the time that faecal bacteria are exposed to light-mediated factors, the more rapid their die off (Mara, 2003).
High pH: rapid algal photosynthesis results in hydroxyl ions (OH) accumulation (as shown former in the chemical formulas provided in Section 3.1.1.2) and thus this rises the in-pond PH usually to larger than 9.4. Such pH values are rapidly lethal to all faecal bacteria with exception to Vibro cholera which can tolerate high pH values (Mara, 1992; Mara, 2003). However Vibro cholera are too sensitive to low concentrations of sulphide > 3 mg/l and thus most of their die-off occurs in anaerobic ponds (Arridge et al. 1995).
High dissolved oxygen and high light intensity: light having wavelengths up to 700 nm are effective in damaging faecal bacteria (Curtis et al. 1992). Curtis also found that lights of wavelengths > 425 nm is able to damage faecal bacteria only in the presence of both dissolved oxygen and dissolved chemical sensitizer (i.e. humic substance gilvin), which both of these parameters are important for the photo-oxidation process that result in bacterial die-off . This process is enhanced by high in
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– pond pH values (pH > 7.7). Thus, algae are essential for bacterial die off as they provide high dissolved oxygen concentrations and induce high in-pond pH which together with high light intensities results in rapid faecal bacteria die-off. The mechanism of this process in damaging the bacteria is that oxygen radicals (i.e.
hydrogen peroxide) are produced which then, with high pH, damage the cell membrane causing cell die-off
Given these light -mediated factors illustrated above, the sun thus has a threefold role. Figure 3.5 below shows a diagram that illustrates the sun role in promoting bacterial die-off in WSP.
Protozoan cysts and helminth eggs: cysts and eggs are mainly removed by sedimentation with the former being removed slower than the later as their sizes are smaller. This sedimentation is mainly occurred in anaerobic or primary facultative ponds (Mara, 1992; Mara, 2003).
Figure 3. 5 Conceptual mechnaism for faecal bacteria die-off. Source: Mara et al. (2003)
Design of maturation ponds:
Maturation ponds are normally designed for E.coli removal and human intestinal helminth eggs removal. The approach of Marais (1974) is widely used in designing a series of ponds for E.coli removal. Marais approach is based on the assumption that E.coli removal can be mathematically modelled by a first order kinetics in a completely mixed reactor and thus the effluent from a single pond can be expressed in the following equation;
=
( )v V(3.18)
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Where, is the number of E.coli per 100 ml of the effluent.
T is the number of E.coli per 100 ml of the influent.
@
is the hydraulic retention time, days.() is the first order rate constant for E.coli removal in a completely mixed reactor at z ℃, expressed in day. This coefficient is highly temperature dependent and its variations with temperature are expressed by Marais empirical formula as follow;
() = 2.6(1.19) (3.19) Where, T ℃ is the design temperature which is either the mean temperature of the coldest month in the season that the maturation ponds are required to produce effluent of a desired microbiological quality (i.e. the irrigation season if the effluent to be used in crop irrigation);
or the mean temperature of the coldest month if the effluent to be discharged in a water body or used for fertilizing fishponds.
For a typical series of WSP starting with anaerobic pond, followed by a secondary facultative pond and number of equally sized maturation ponds and with the effluent of a pond will be the influent to the subsequent one, equation 2.44 can be rewritten as follow (Mara,2003);
=
¡( )Vy¢¡( )v V¢(( )V)£(3.20)
Where, T and are now the number of E.coli per 100 ml of the raw wastewater and final effluent, respectively;
The subscripts N, ¤ and n refers to the pond type; anaerobic, facultative and maturation
Q is the number of equally sized maturation ponds, if the maturation ponds are not equally size the term (1 + ()@m)U would be replaced by (1 + ()@m)(1 +
()@m)... (1 + ()@mU).
Mara (2003) recommends that the hydraulic retention time of maturation pond (@m) should be subject to the following; to be not larger that of facultative pond (@l) , not less than the acceptable minimum retention time (@mmTU) which is 3 days in order to allow for sufficient time for algal reproduction and to avoid hydraulic short- circuiting , and that the first
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maturation pond should not have a surface BOD loading rate *S(m) larger than 75 percent of that in facultative ponds *S(l). The latter constraint is the critical one to be considered first in the design;
The BOD surface loading rate for the first maturation pond is given by following equation;
*
S(m)=
uvw(3.21) By substituting Q in equation 3.21 with (A× Om/@m)and setting *S(m) = 0.75 *S(l) ,equation 3.21 becomes as follow:
0.75 *S(l) = ¥¦§¨
V
(3.22) Rearranging equation 3.22 for the value of @m
,
the equation becomes as follow,@
m=
.© ¥¦ª()§(3.23)
Where,
T is the unfiltered BOD in the flow entering the first maturation pond – which is equivalent to that of facultative pond effluent ( (l)) as calculated from equation 3.12.
Equation 3.20 can now be rewritten as;
=
¡( )Vy¢¡( )V¢(v ( )V)(( )V)£(3.24)
Where, @m now refers to the retention time in maturation ponds after the first one
All the parameters in equation 3.24 are known or assumed at this stage of the design except n and @m; the term can be assumed depending on the required quality of the final effluent.
Equation 3.24 can be rearranged for the value @m to ease solving it, thus the equation becomes:
@m = {[T/¡1 + ()@¢¡1 + ()@l¢(1 + ()@m)]/U− 1}/() (3.25) Equation 3.25 is solved for Q = 1, for Q = 2 and so on, till the obtained value of @m is less than @mmTU. Ignore the values of @m which are > @l and< @mmTU; and then choose the combination of @m and Q, including @mmTU and Q¯ (Q¯ is the value of n of which @m first
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becomes less than @mmTU ), which gives the least overall retention time and thus the minimum area requirement. In other words, a combination of which their product (@m× Q) is a minimum (Mara, 2003).
The number of eggs then can be checked from the following equation;
No. of eggs in the inal efluent = [no. of the eggs in facultative pond efluent ×(1-²m1)(1-²m )Q] (3.26) Where ²³ and ²³ are the percentage of egg removal achieved in the first maturation pond and subsequent ponds, respectively. These percentages are calculated from equation 3.16.
The area of maturation pond, taking in consideration the net evaporation, can be calculated from the following equation (derived from equation 3.11 by using m as subscript and rearranging the equation for m);
m=
K wvV.V
(3.27)
Where, is the net evaporation in mm/day.
T is the influent flow in m /day , this value is equal to the effluent flow leaving the pond preceding the considered one.
After calculating the maturation pond area, the effluent flow can be calculated from the following equation;
= T − 0.001 l (3.28) Pond effluent polishing
Compliance with strict quality requirements in term of suspended solid levels (i.e. less than 50 mg/l) requires removing the algal suspended solids from the effluent (Mara, 2003). The environmental protection agency (2002) and Mara (2003) strongly recommend the use of rock filters unit for this purpose. These filters consist of submerged bed of rocks through which the pond effluent flows horizontally and thus algae settle out. Rock filters can be either in–pond filters in the last maturation pond or a separate unit after the last maturation pond (the latter is operationally better). Rocks of size (50 to 200 mm) are used in rock filters. The rocks should typically extend at least 300 mm above the liquid level in the filter in order to minimize problems of odour and mosquitoes breeding. Rock filters design are based on
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hydraulic loading rate (HLR) which is defined as volume of pond effluent in n applied to the gross rock filter volume in n per day, giving it an overall unit of d(Mara, 2003). HLR ranges from 0.25 to 1.2 d, with low rates are more recommended (Environmental Protection Agency, 2002).