The six natural wastewater treatment systems, considered in this study (see Figure 5.1), are designed to treat a proportion of Gaza Strip’s wastewater (17000 m /day) with its characteristics as given above, and to produce an effluent that contains a BOD of no more than 30mg BOD/l; FC per 100 ml of no more than 0.01 of that in the influent and number of helminth eggs of no more than 0.10 of that in the influent. Thus, land areas requirements of NWWT systems can be determined from which the applicability of NWWT in the Gaza Strip in term of land area requirements can be assessed and from which also they can be compared in term of land area requirement. The design inputs and the design of the different NWWT units are as follow;
Design inputs:
-The flow rate of the influent ( T) is 17000 m /day.
-The influent has a BOD of 667 mg/l, T of 10©-10Z FC per 100 ml and 10-100 eggs per litre.
-The design temperature and the corresponding evaporation rate were taken 22 ℃ and 8 mm/day, respectively.
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(a) Anaerobic pond:
Following the design procedures of anaerobic pond illustrated in chapter 3, the volumetric BOD loading (*t , g/m .day), based on table 3.2, is given by; *t = 10z + 100
*t = 10(22) + 100 = 320 g/m . day The pond volume # is given by equation 3.1 as:
#
=
uvw =
(kk©)×(©)
=
35435 m The retention time is given by equation 3.2 as:@
=
xwy=
8 ©=
2.09day > 1 day okBased on a pond depth of 4 m, the pond area is given from equation 3.3 as;
=
Kxyy=
8 8=
8858.75 mThe BOD removal is given from Table 3.2 as; ² = 2z + 20 = (2)(22) + 20 = 64 %
The BOD in the effluent is given from equation 3.4 as;
=T× [1 −R (%)]= 667 × [1 −.64] = 240.12 mg/l > 30 mg/l The first order rate constant for FC removal is given from equation 3.19 as; () = 2.6(1.19) =2.6 (1.19) = 3.68 day
The FC in the effluent is given from equation 3.18 as;
=
v( )Vy =
v
( .kZ)(.Ñ)
=
0.115 T FC per 100 ml > 0.01TThe percentage of eggs removal in anaerobic pond is given from equation 3.16 as; R= 100[1 − 0.41 exp¡−0.49θ+ .0085 θ¢]= 100[1 −
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The no. of eggs in the effluent thus is given by equation 3.15 as;
No.of eggs in the efluent = No of eggs in inluent P1 − R>
= No of eggs in inluent P1 − 0.8472>=
0.1528× No of eggs in inluent Pper litre> > 0.10 × No of eggs in inluent. The BOD, FC per 100 ml and No. of eggs in the effluent of anaerobic ponds indicate that a secondary treatment unit is required. This could be, as shown above, either facultative pond or constructed wetland.
Drying beds is required for the sludge produced; Based on drying bed area of 0.015
m/person and estimated population to be served of 265625 person (this was calculated based on domestic water consumption 80 l/c/d and a return factor of 0.80) , the area of drying beds can be given as follow;
Total area of drying beds = number of population served × area of drying bed per person = 0.015 × 265625 = 3984.5 m.
(b)UASB reactors:
As shown in the literature review (chapter 3), UASB reactor can replace anaerobic pond. Following design procedures of UASB in the Chapter 3, the volume of UASB reactor, using a retention time of 6hr, is given by equation 3.30 as;
# = T × @ = 17000 × P6/24> = 4250 m
Since the Volume of the UASB reactor is recommended to be not greater than 1000
m , the number of UASB – 1000 m reactors is given by; No. of UASB reactors = 8 ³
Ò
³Ò≈ 5 reactors. Assuming a reactor depth of 6 m, the total area required thus;
Total area of the reactors = ³ Ò
k ³ × 5 =834 m
The flow rate for each of the UASB reactor is 4000 m /day except the fifth reactor having less than that, thus the upward velocity in each of the four reactors is given from equation 3.31as ;
Chapter (5) Applicability of Natural Wastewater Treatment and Wastewater Reuse in the Gaza Strip 97
"
µg=
wv =
8/8 (Z 8/)=
0.99 m/hr < 1m/h ok. The COD removal in UASB is given by equation 3.29 as; ² = 100(1 − @.kZ)= 100(1 − 6.kZ)= 70.5 % The COD in the effluent thus is given from;COD in the effluent = COD in the influent × (1−² %) = 1306× (1−0.705) = 385.27 mg/l.
Since BOD/COD ≈ 0.50 (Mara, 2003), the BOD in the effluent of UASB is given as follow;
=ÔKÕ
ÖÔK × COD in the effluent = 0.50 × 385.27 mg/l = 192.64 mg/l > 30 mg/l UASB reactors achieve FC removal of about 80 percent (Van Haandel and Lettinga,
1994), thus the FC in the effluent can be calculated as follow; = T[1 − (80/100)]= 0.20 T > 0.01 T
UASB reactors achieve helminth eggs removal of about 82 percent (Van Haandel and Lettinga, 1994), thus the no. of eggs in the effluent can be calculated as follow;
No.of eggs in the efluent = No of eggs in inluent P1 − R>
= No of eggs in inluent P1 − 0.82>=
0.18× No of eggs in inluent Pper litre> > 0.10× No of eggs in inluent.
The UASB reactors similarly as anaerobic pond should be followed by either secondary facultative pond or constructed wetland to achieve effluent of a quality described above. The UASB reactors also require drying beds for drying the sludge produced.
(c) Secondary facultative pond
Secondary facultative pond follows either one of the primary treatment units designed above. Following the design procedures of facultative ponds in chapter (3), the design of facultative pond is as follow;
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The surface BOD loading ( *S , kg BOD / ha.day) is given from equation 3.7 as;
*S = 350(1.107 − .002z) =350[1.107 − .002(22)] = 291.386 kg BOD / ha.day.
The surface area of facultative pond is given by equation 3.6 as;
l=
uvwv ª=
uv(©)
Ñ. Zk = 583.420 T m
The surface areas of facultative ponds for the different NWWT systems are thus as follow;
T (mg/l) l(m)
Facultative pond after anaerobic pond : 240.12 140090.6 Facultative pond after UASB reactors: 193.09 112656 Facultative pond after HRAP: 143.66 83814
Assuming facultative pond depth of 1.5 m, the retention time of facultative pond is given by equation 3.9 as;
] 008 . 0 34000 [ 3 ] ) 8 ( 001 . 0 ) 17000 ( 2 [ ) 50 . 1 ( 2 ] 001 . 0 2 [ 2 f f f f f i f f A A A A eA Q D A − = − = − = θ
Thus, the retention time of the facultative ponds for the different NWWT systems are as follow:
l(m)
f
θ
(
day) Facultative pond after anaerobic pond : 140090.6 12.78 > 5 ok Facultative pond after UASB reactors: 112656 10.21 > 5 ok Facultative pond after HRAP: 83814 7.54 > 5 okThe effluent flow from facultative pond is given by equation 3.10 as;
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Thus, the effluent flow rates from the facultative ponds for the different systems are as follow;
l(m)
(
m /day)Facultative pond after anaerobic pond : 140090.6 15879.28 Facultative pond after UASB reactors: 112656 16098.76 Facultative pond after HRAP: 83814 16329.49
Using () = 0.10 d for secondary facultative pond, the first-order rate constant for BOD removal is given by equation 3.13 as;
() = ()(1.05) = (0.10) × (1.05) = 0.110 d
The Unfiltered BOD in the effluent of facultative pond is given from equation 3.12;
= 1 + T @l
=
T
1 + 0.11 × @l
Thus, the unfiltered BOD in the effluents of facultative ponds for the different NWWT systems is given as follow;
T (mg/l)
@
l(
day) (µUl.) (mg/l)Facultative pond after anaerobic pond : 240.12 12.78 99.66 Facultative pond after UASB reactors: 193.09 10.21 90.84 Facultative pond after HRAP: 143.66 7.54 78.43
The filtered BOD is given from equation 3.14 as follow (using non algal fraction
U=0.30);
(iltered) = U × [(unfiltered) ]= 0.30 × [(unfiltered)]
Thus, filtered BOD from in the effluent of facultative ponds for the different NWWT systems is given as follow;
(×Ul.) (mg/l) (ØTbÙ.) (mg/l)
Facultative pond after anaerobic pond : 99.66 29.89 < 30 mg/l ok Facultative pond after UASB reactors: 90.84 27.25 < 30 mg/l ok Facultative pond after HRAP: 78.43 23.53 < 30 mg/l ok
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The first order rate constant for FC removal is given from equation 3.19 as; () = 2.6(1.19) =2.6 (1.19) = 3.68 day
The FC in the effluent of facultative pond in the effluent is given from equation as;
=
v[]( )V =
v[]
.kZV
Note: Ne[¤]in the equation above is the NÚin the effluent produced from the primary treatment (i.e. anaerobic pond, UASB reactors or HRAP).
The FC in the effluent from the different natural wastewater treatment systems thus is given as follow;
T[l] (FC per 100ml) @l(day) (FC per 100ml) Facultative pond after anaerobic pond : 0.115T 12.78 0.0024 T
Facultative pond after UASB reactors: 0.20T 10.25 0.0051 T Facultative pond after HRAP: 0.30T 7.54 0.0099
All the systems above shown produce an effluent that contains no more than 0.01T FC per 100 ml, where T is the FC per 100 ml in the influent. Thus, these systems achieve the 2-log unit reduction of pathogen (expressed in FC per 100 ml) required to protect the health of those working in the wastewater-irrigated fields.
The percentage of eggs removal in secondary facultative pond is given from equation 3.16 as;
R= 100[1 − 0.41 exp¡−0.49θ+ .0085 θ¢]
Thus, egg removals in the Facultative ponds of the different natural systems are given as follow;
@
l(
day) R %Facultative pond after anaerobic pond : 12.78 99.68 Facultative pond after UASB reactors: 10.21 99.33 Facultative pond after HRAP: 7.54 98.35
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The no. of eggs in the effluent from in secondary facultative pond is given from equation 3.15 as;
No.of eggs in the efluent = No of eggs in inluent × Û1 −100ÜR
Note: Number of eggs in the influent in the equation illustrated above is the number of eggs in the effluent from the primary treatment system. The number of eggs in raw wastewater will be given a symbol (EG) to facilitate the presentation. Thus, the numbers of eggs in the effluents from the facultative ponds for the different NWWT systems are given as follow;
No of eggs per litre of
influent
R % No of eggs per litre of effluent
Facultative pond after anaerobic pond : 0.1528× EG 99.68 0.00049× EG Facultative pond after UASB reactors: 0.180 × EG 99.33 0.0012 × EG Facultative pond after HRAP: 0.150× EG 98.35 0.0025 × EG
The number of helminth eggs in the effluent of facultative ponds from the three different natural treatment systems shown above is less than 0.1 EG. Thus, these systems achieve the 1-log unit reduction for Ascaris required to protect the health of those working in the wastewater-irrigated fields. Since they achieve required log-unit reduction of pathogens (both faecal coliform and helminth eggs), there is no need for these systems to be followed with maturation ponds.
Note: all the systems above provide a factor of safety to the scheme as they achieve greater log unit reduction of pathogens than what are required.
(d)Constructed wetland:
Constructed wetland is a secondary treatment unit follows either one of the primary treatment units designed above (i.e. anaerobic pond, UASB reactors or HRAP). It was assumed that porosity of the planted gravel bed is 0.40 (i.e. 25-mm gravel is used) and the depth of the bed is 0.60m. Following the design procedures of constructed wetland in Chapter (3), the design of facultative pond is as follow;
The value of ¾ is given from equation 3.36 as;
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The area of constructed wetland is given by rearranging equation 3.35 as; ¼½ = [−2 Tln (/T)]/[2¾3 O¼½− (0.001 Q(/T))]
The constructed wetland following each of the primary treatment unit is designed to produce an effluent that contains no more than 30 mg /l (i.e. = 29 mg/l), thus the area of each constructed wetland in the three systems is given from the following equation;
¼½ = [−2(17000)ln (29/T)]/[2(1.69)(0.40)(0.60) − (0.001(8)Q(29/T))] ¼½ = [−(3400)ln (29/T)]/[0.8112−0.008Q(29/T))]
Note: T is the influent to the constructed wetland, which is the effluent from the primary treatment unit (i.e. anaerobic pond, UASB reactors or HRAP)
Thus the area of each constructed wetland in each of the three natural treatment systems is given as follow;
T (mg/l) ¼½ (m)
Constructed wetland after anaerobic pond : 240.12 87015.40 Constructed wetland after UASB reactors: 193.09 78207.38 Constructed wetland after HRAP: 143.66 66198.9
Constructed wetlands can achieve 2-log unit reduction of total coliform (Van Haandel and Lettinga, 1994), and they are very efficient in term of helminth eggs removal as they can produce effluent with no eggs (Stott et. al. 1999). These three NWWT systems, having constructed wetlands as a secondary treatment unit, are considered to produce effluent that is considered safe (based on the analyses done in section 5.1) for the people working in the wastewater-irrigated field.
Given the design of each unit of the different NWWT systems above, Table 5.6 below summarises the total lands area requirements for each system to treat 17000 m /day of the Gaza Strip’s wastewater and to produce an effluent having characteristics as described above. An important finding can be extracted from the results shown in Table 5.6 is that the land area requirements for the different systems are much less than the area of the existing treatment plant in the north governorate (Biet lahia wastewater treatment plant - consisting of aerobic, polishing and infiltration ponds) that has an area of 400000 m and treats
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approximately the same proportion of Gaza’s wastewater (17000 m /day) (EWASH, 2009). Thus, this together with the more areas that became available in the Gaza Strip (due to the withdrawal of Israel militaries who were occupying 40% of the Gaza Strip) indicates that natural wastewater treatment is applicable in the Gaza Strip in term of land area requirement.
Table 5. 6: Lands area requiremnts for the six proposed NWWT systems to treat 17000 m3/day of Gaza’s wastewater
System code System description Units area (m) S1 Anaerobic pond followed by
facultative pond
Anaerobic pond: 8858.75 Facultative pond 140090.6 Drying beds 3984.50
Total area= 152933.85 m S2 Anaerobic pond followed by
constructed wetland
Anaerobic pond 8858.75 Constructed wetland 87015.40 Drying beds 3984.50
Total area= 99858.65 m S3 UASB reactors with drying
beds followed by facultative pond
UASB reactors 834 Facultative pond 112656 Drying beds 3984.50
Total area= 117474.50 m S4 UASB reactors with drying
beds followed by constructed wetland
UASB reactors 834 Constructed wetland 78207.38 Drying beds 3984.50
Total area= 83025.88 m S5 High rate anaerobic ponds
followed by facultative pond
HRAPs 3190
Facultative pond 83814
Total area= 87004 m S6 High rate anaerobic ponds
followed by constructed
HRAPs 3190
Constructed wetland 66198.90
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As shown in Table 5.6, system S1 (anaerobic pond followed by facultative pond) has the largest lands area requirement and S6 (HRAPs followed by constructed wetland) has the least lands requirement among the six systems. However, to find out the most suitable NWWT system for the Gaza Strip among the six NWWT systems, land area requirements should be considered together with other requirements (i.e. construction, costs, operation and maintenance).