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Fertilizing value and polluting load of reclaimed water in Tunisia
PII: S0043-1354(98)00135-3
Wat. Res. Vol. 32, No. 11, pp. 3484±3489, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
TECHNICAL NOTE FERTILIZING VALUE AND POLLUTING LOAD OF RECLAIMED WATER IN TUNISIA M AKISSA BAHRI**
National Research Institute for Agricultural Engineering, Water, and Forestry, B.P. 10, Ariana 2080, Tunisia (First received September 1996; accepted March 1998) AbstractÐA national survey of wastewater composition was conducted for 28 chemical constituents from 15 treatment plants located across Tunisia to provide a basis for establishing water quality and management for crop production and environmental protection. The contents of salt, nutrients and trace elements were investigated and showed that the composition was characterized by a moderate salinity and sodicity for most of the plants, a high variability of the organic parameters, and a low trace elements content. In the case of N and P, such variations may be a constraint in using the water for fertilizing purposes. An increase in salinity with time was noticed for several plants. While the salt contents of wastewater were not aected by the treatment process, total N and P removal eciencies were 48% (62% for ammonium) and 63%, respectively. For trace elements, removals averaged: Zn 87%; Cu and Fe 78%; Pb 64%; Cr 50%; Mn 44%; Co and Cd 17%; and Ni 13%. The analyzed reclaimed water samples were consistent with the Tunisian standards regarding water quality required for agricultural reuse and had a high fertilizing content. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐwastewater quality, salinity, nutrients, trace elements, removal eciency
INTRODUCTION
MATERIALS AND METHODS
Physical, chemical, and biological parameters are important when characterizing wastewater for design of reuse facilities and in the management of environmental quality (Metcalf and Eddy, 1991). Municipal treatment plants usually have been designed to remove organic matter and suspended solids, but other wastewater constituents may be of concern for dierent reuse objectives (Asano et al., 1985) and thus have to be monitored.
The municipal wastewater was mainly domestic (80%) and processed through biological treatment plants up to the secondary level. The survey was done for the 15 following types of plants:
Many studies have been conducted on wastewater quality and its suitability for dierent purposes (Nielsen and Hrudey, 1983; Pettygrove and Asano, 1985; Akhter, 1991; Chin and Ong, 1991; Lauer, 1991; Metcalf and Eddy, 1991; Crook et al., 1992). In the Mediterranean area, such studies are rather few. In Tunisia, a survey over most of the sewage treatment plants supplying irrigation water was conducted to examine variation in water quality for individual treatment plant as well as between dierent treatment plants, removal eciencies, and to evaluate the fertilizing value and polluting load of municipal wastewater euents.
Besides these plants, reclaimed water used for irrigating an area (3200 ha) located 20 km north of Tunis which are Cherguia, CoÃtieÁre Nord, and Choutrana euents blended together has been analyzed. Whereas for some treatment plants (Cherguia, SE4, Choutrana, CoÃtieÁre Nord, Sud MeÂliane, and Sfax) the monitoring was done for several years, other plants and in¯uents were observed during much shorter periods. The sampling frequency was once a week for the plants located in the Tunis area; later, it was changed to once a month for all locations. Grab samples were taken from the stabilization ponds and composite samples (2 h) from the other treatment processes. Standard techniques were used to analyze the dierent parameters: pH, electrical conductivity (measured at 258C), total dissolved solids (TDS, evaporation at 1058C), suspended solids (SS) (®ltration on glass-®ber ®lter disk (Afnor, 1979)), chemical oxygen demand (COD) (potassium dichromate method (Afnor, 1979)), biochemical oxygen demand (BOD5) (incubation method (Afnor, 1979)), Na and K (¯ame photometric method), Ca and Mg
*[Tel: 216 1 719630, Fax: 216 1 717951].
. activated sludge: Cherguia, SE1, SE2, SE4, Sousse Nord, and Kairouan; . trickling ®lter: Monastir; . activated sludge + trickling ®lter: Sousse Sud; . oxidation ditch: Choutrana, Sud MeÂliane, and SE3; . stabilization ponds (SP) and aerated lagoons (AL): CoÃtieÁre Nord (SP), Moknine (SP), Gafsa (SP), and Sfax (AL).
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Technical Note (EDTA titrimetric method), Cl (argentometric method), SO4 (turbidimetric method), HCO3 (titrimetric method), Kjeldahl nitrogen (NK), NH4 (distillation or phenate method), NO3 (reduction through Cd±Hg column), NO2 (sulfanilamide diazotization), total phosphorus (P) (Na2S2O8±H2SO4 mineralization, Murphy and Riley (1962) colorimetric determination), phosphate ions (PO4) (spectrophotometric method (Afnor, 1979)), B (colorimetric method Ð azomethin H), trace elements (Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, and Zn) by atomic absorption spectroscopy (Perkin Elmer 2380 atomic absorption spectrophotometer) after concentrating 1 l of sample (with addition of HNO3). A geochemical characterization and statistical analysis were performed on the data.
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was less than the standard (30 mg lÿ1) in most of the treatment plants except in the lagoons, and in one trickling ®lter (Monastir). Minimum (1 mg lÿ1) and maximum values (357 mg lÿ1) were observed at Cherguia and Moknine treatment plants, respectively. The average value for the in¯uents was 360 mg lÿ1. Water discharged from the lagoons had much higher COD, BOD5, and SS content compared to the other processes because of algae development. For other plants, high values were due to overloading. Salt content
RESULTS AND DISCUSSION
The content of salts, fertilizers, and trace elements for wastewater samples is discussed in this paper. Data for euent from a given treatment plant (not shown) and for raw and reclaimed water from multiple plants (Tables 1 and 2) are reported. General characteristics Based on samples analyzed, the quality of euent was more or less consistent with the Tunisian standards for agricultural reuse (Table 2) (INNORPI, 1989). The pH of euent varied between 7.5 and 7.9 (with minimum value (6.8) observed at Monastir, SE3, and Sousse Nord, and maximum value (8.8) at CoÃtieÁre Nord). The average content of suspended solids was 42.5 mg lÿ1 (varying between 15 and 191 mg lÿ1). The maximum average value was found in Moknine (aerated lagoons). It
The minimum and maximum EC were respectively observed in Cherguia (1.16 dS mÿ1) and CoÃtieÁre Nord (12.63 dS mÿ1) plants. The average electrical conductivity of reclaimed water varied between 2.4 and 8.9 dS mÿ1 and that of the total dissolved solids was between 1.5 and 5.6 g lÿ1. In¯uents and euents had the same salinity range. Treatment processes did not aect the water salinity except for stabilization ponds in which salinity increased during warm periods. The in¯uent salinity was, in the case of CoÃtieÁre Nord, due to seawater seepage into the collection network, and at the plant location (near a salt lake). Sfax water supply, more mineralized than in the other cities, was partly responsible for the higher than normal salinity in its wastewater. For Moknine, the industrial activities (50% of the sewage water) and the plant location (in a salt lake), were both responsible for the sal-
Table 1. Descriptive statistics of average element concentration for in¯uents from 15 wastewater treatment plants in Tunisia (in mg lÿ1 unless otherwise indicated) Parameter
Mean
Median
SD
Min.
Max.
pH EC (dS mÿ1) TDS (g lÿ1) Ca Mg K Na HCO3 SO4 Cl SAR SS COD BOD5 Nk NH4-N NO3-N NO2-N P PO4-P Cd Co Cr Cu Fe Mn Ni Pb Zn
7.6 4.72 2.95 167 91 51 622 652 515 856 9.4 359.0 558.8 248.6 76.7 67.4 0.48 2.62 9.43 6.17 0.006 0.023 0.032 0.079 1.017 0.096 0.039 0.122 0.280
7.7 4.27 2.56 161 78 49 507 641 476 707 7.7 307.5 479.9 267.2 67.2 63.9 0.06 0.07 9.80 2.53 0.006 0.022 0.024 0.073 0.915 0.089 0.038 0.106 0.290
3.3 1.85 1.13 34 39 23 370 146 157 640 4.0 179.1 186.0 66.7 25.7 20.8 0.68 3.80 2.09 1.73 0.002 0.007 0.025 0.030 0.646 0.050 0.008 0.078 0.124
7.1 2.99 1.92 129 62 19 284 342 266 254 4.9 232.0 396.6 69.4 41.9 33.4 0.02 0.01 3.40 2.74 0.004 0.012 0.010 0.040 0.199 0.033 0.027 0.050 0.074
7.9 10.4 6.59 262 219 101 1790 888 840 2955 20.8 765.6 933.0 325.7 141.6 100.0 1.81 11.17 12.20 9.04 0.010 0.041 0.109 0.160 2.780 0.230 0.051 0.347 0.482
CV 0.033 0.392 0.383 0.205 0.429 0.455 0.596 0.224 0.305 0.746 0.431 0.499 0.333 0.268 0.335 0.308 1.425 1.452 0.222 0.280 0.333 0.304 0.781 0.380 0.635 0.521 0.205 0.639 0.443
SD: standard deviation; CV: coecient of variation; n: number of treatment plants; SAR: sodium adsorption ratio.
n 15 15 15 15 15 15 15 15 15 15 15 8 7 14 15 15 11 15 15 13 15 15 15 15 15 15 15 15 15
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Technical Note
Table 2. Descriptive statistics of average element concentration for euents from 15 wastewater treatment plants in Tunisia (in mg lÿ1 unless otherwise indicated) Parameter
Mean
Median
SD
Min.
Max.
pH EC (dS mÿ1) TDS (g lÿ1) Ca Mg K Na HCO3 SO4 Cl SAR SS COD BOD5 Nk NH4ÿN NO3-N NO2-N P PO4-P Cd Co Cr Cu Fe Mn Ni Pb Zn
7.6 4.10 2.61 168 85 52 537 524 532 791 8.5 42.5 173.6 35.3 30.0 26.2 9.5 2.48 3.60 2.34 0.005 0.019 0.016 0.017 0.226 0.054 0.034 0.044 0.036
7.6 3.42 2.23 171 72 44 431 473 484 625 7.2 20.0 103.2 26.8 26.5 23.7 7.2 1.80 3.30 1.90 0.005 0.016 0.016 0.016 0.181 0.048 0.033 0.042 0.033
0.1 1.68 1.08 31 36 27 293 189 147 526 3.8 47.9 152.7 18.4 11.0 10.6 6.0 2.19 1.63 1.08 0.001 0.006 0.004 0.004 0.115 0.025 0.009 0.008 0.011
7.5 2.39 1.52 121 54 18 293 333 304 338 5.1 14.7 61.4 17.8 16.9 14.4 2.1 0.52 1.61 1.23 0.004 0.012 0.009 0.011 0.108 0.022 0.021 0.035 0.023
7.9 8.94 5.61 238 188 120 1438 1046 922 2490 17.6 190.9 639.5 69.8 53.1 48.3 23.2 8.89 6.54 4.34 0.008 0.031 0.023 0.025 0.511 0.112 0.049 0.066 0.063
CV
NT
0.017 0.410 0.414 0.185 0.427 0.512 0.553 0.360 0.277 0.666 0.449 1.128 0.879 0.521 0.367 0.403 0.631 0.881 0.453 0.462 0.240 0.337 0.256 0.251 0.507 0.470 0.267 0.177 0.316
6.5±8.5 7.0 ÿ ÿ ÿ ÿ ÿ ÿ 2000 ÿ 30 (a) 90 (a) 30 (a) ÿ ÿ ÿ ÿ ÿ ÿ 0.01 0.1 0.1 0.5 5 0.5 0.2 1 5
SD: standard deviation; CV: coecient of variation; NT: Tunisian standards; (a): except special authorization. SAR: sodium adsorption ratio.
inity of the wastewater. The high salt content of wastewater at Sud MeÂliane may be the result of high density of industrial units. For both the raw and euent, Na and Cl were (in Piper (1944) diagram) the prevailing mineral ions except for wastewater from Kairouan and Gafsa in which HCO3 or SO4 was in greater concentration over Cl. Dierences between in¯uent and euent composition were essentially due to geochemical processes occuring during the treatment such as mineral precipitation (calcite). The SO4 concentrations were always higher than those of Ca. The chemical equilibrium model (SIMEQ) based on the Pitzer equations (speci®c ionic interaction model) (Gueddari, 1984) was used to test saturation index of some mineral salts. Almost all samples were over-saturated with respect to calcite except a few samples at Sousse Nord samples (with ionic strength less than 0.03 mol lÿ1) and under-saturated with respect to gypsum. The average value of the sodium adsorption ratio (SAR) of euents ranged between 8.7 and 17.6 at CoÃtieÁre Nord, Moknine, Sud MeÂliane, Sfax, SE1, and Monastir. The SAR of the other treatment plants varied between 5.1 and 7.4. The minimum average value was found for Gafsa and the maximum for CoÃtieÁre Nord.
n = 15) and varied generally between 27 mg lÿ1 for SE3 and 85 mg lÿ1 for Gafsa. Nitrogen was essentially present as ammonia except at SE2 where nitrate was prevailing. However, it was noted that in most of the plants and during certain periods, nitrates and sometimes nitrites were inversely related to ammonia in the euent (Fig. 1). In the in¯uent, the average total nitrogen content was 80 mg lÿ1 (CV = 0.35, n = 15) with a minimum value (44 mg lÿ1) for SE3, and maximum value (149 mg lÿ1) for Gafsa. The total nitrogen of the in¯uent consisted of essentially ammonia and small amounts of nitrate or nitrite. The average total phosphorus concentration in the euents was 3.6 mg lÿ1 (varying between 1.6
Nutrient content The average total nitrogen content (Norg+NH4+NO3+NO2) of the euent was about 42 mg lÿ1 (coecient of variation (CV) = 0.38,
Fig. 1. Variation with time of NH4 and NO3 concentrations in CoÃtieÁre Nord euents from 1988 to 1989.
Technical Note
and 6.5 mg lÿ1). Phosphorus was essentially in an inorganic soluble form. The average content of PO4 ions (PO4-P: 2.3 mg lÿ1) of euents varied from 1.2 mg lÿ1 for Choutrana to 4.3 mg lÿ1 for Gafsa. The average concentration in the in¯uents for total phosphorus was 9.4 mg lÿ1 with a range from 3.4 to 12.2 mg lÿ1. The average potassium content of the euents varied from 17.5 mg lÿ1 for SE3 to 120 mg lÿ1 for Moknine. The average for all plants was 52 mg lÿ1. The same average value was found in the in¯uents. A comparison of the fertilizing content in the euent showed that, in all the water samples, the amount of phosphorus was much less than that for nitrogen and potassium: N=P2 O5 =K2 O:1=0:2=1:6
N=P=K:1=0:1=1:3:
Trace element content Trace element concentrations of in¯uents and euents were all below the maximum concentrations recommended for agricultural reuse by the Tunisian standards (Table 2) (INNORPI, 1989). In the euents, the highest concentration for most of the trace elements was found for Sfax and the lowest for SE3. The following sequences were common in the in¯uent and euent: influent: Fe Zn > Pb > Mn > Cu > Ni > Cr > Co > Cd effluent: Fe Mn > Pb > Zn > Ni > Cu > Co > Cr > Cd Average boron content was around 0.8 2 0.2 mg lÿ1 in the in¯uent (ranging from 0.3 to 1.7 mg lÿ1) and 0.7 2 0.2 mg lÿ1 in the euent (ranging from 0.3 to 1.4 mg lÿ1). These values were within the usual range in irrigation water (Ayers and Westcot, 1985). Statistical characteristics Descriptive statistics of element concentration in raw and reclaimed water are given in Tables 1 and 2. The data for euent samples obtained from dierent treatment plants (not shown) showed that the extent of temporal variability in composition of euent depended on parameters. Parameters relating to organic matter and trace element content in the water exhibited a considerable variability (CV = 0.40±1.83 on average) with nitrite and nitrate displaying the extreme variability (CV up to 2.50). The CV was lower for the major ions, EC, and TDS (CV < 0.25 on average). The variability of water quality between dierent treatment plants (Tables 1 and 2) was larger for the organic matterrelated parameters compared to the inorganic ones (major and minor elements). The CV for the trace
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elements of reclaimed water from dierent treatment plants was smaller than that for a given treatment plant and the contrary was the case for the major ions due to the dierent salt contents of the wastewater. The variation of trace element between treatment plants and especially for euents was then within a relatively narrow range (CV about 0.30 except for Fe and Mn). The Kolmogorov±Smirnov test was used to ®t theoretical probability distribution functions (pdf) to the dierent parameters in the euent. Theoretical normal and/or lognormal pdfs could, in several cases, be ®tted to the parameters except Cd. The suspended solids, COD, BOD5, NO3, and NO2 had more skew distributions that followed lognormal distributions. However, as the number of data increased, as in the case of Cherguia and SE4, the parameters showed poor accordance with the normal or lognormal pdfs. This may be due to the presence of several outliers creating discontinuities in the empirical pdfs. The variability in the euent quality may also reveal some inherent in-plant treatment problems: The Cherguia plant was renovated at the end of the study period. The minimum number of samples needed for mean value estimation with a relative error margin of 10% at a probability level of 0.05 was calculated for the dierent parameters and treatment plants. It was found that this number varied widely from 3± 30 for parameters related to the salt content (EC, TDS, Ca, Mg, Na, K, Cl, HCO3, and SO4), 6±66 for SS, COD, and BOD5, 5±300 for trace elements, and 17±>500 for the dierent nitrogen and phosphorus forms. Process removal eciency A comparison of average sewage in¯uent and euent composition was made to investigate the overall removal eciency. The salt contents of wastewater were not aected by the treatment process as expected. Average COD and BOD5 removal eciencies were 69 and 86%, respectively, while that of total N was 48% (62% for NH4) and that of total P was 63%. For trace elements, the most signi®cant removal was found for Zn (87%), followed by Cu and Fe (78%), Pb (64%), Cr (50%), Mn (44%), Co and Cd (17%), and Ni (13%). These removal eciencies are similar to that found by WPCF (1989) for some secondary treatment processes. Other removal eciencies can be found in the literature (Brown et al., 1973; Oliver and Cosgrove, 1975; Nielsen and Hrudey, 1983) depending on treatment process, loading rate, and in¯uent quality. In this study, Zn, Cu, Fe, and Pb can be assumed to have high contents in the sewage sludge. Substantial amounts of C, N, and P should also be expected in the sewage sludge. On the other hand, the removal eciency depended on the type of treatment process. For example, activated sludge, stabilization ponds, and
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Technical Note
trickling ®lter were more ecient in Nk removal (60±70%) compared to the oxidation ditch (51%). For P, activated sludge and oxidation ditch had higher removal eciencies compared to stabilization ponds. For Cu, Fe, Pb and Zn, higher removal eciencies were obtained with the oxidation ditch compared to the activated sludge process or to the stabilization ponds. Quality changes versus time The quality changes with time for the most important treatment plants (Cherguia, Choutrana, CoÃtieÁre Nord, Sud MeÂliane, and SE4) were evaluated. The composition of euent at a treatment plant varied with time (increase of salinity in Cherguia and Choutrana euents), depending on the eciency of the treatment plant, the proportion of water produced by the dierent activities, etc. Seasonal variations were observed in the CoÃtieÁre Nord water due to long detention time in the lagoons (Fig. 2), on the one hand, and salt reduction from 1988 to 1992 due to the prevention of seawater intrusion into the collection network, on the other hand. Fertilizing value and polluting load of reclaimed water The impacts of euent application on the soil± plant±groundwater system have to be evaluated. In previous studies, in Tunisia, it was demonstrated that the fertilizing units brought by euent had a favourable eect on the growth of certain crops and on their N, P, and K content (Rejeb, 1990). If the N, P, and K average crop uptakes are assumed to be 80 kg N, 45 kg P, and 85 kg K per hectare (Gras and Morisot, 1973), the amount of fertilizing elements present in the monitored wastewater plants may cover the N needs of about 50 000 ha, the P of about 8000 ha, and the K of 60 000 ha. If, at the other hand, 600 to 1000 mm (value corresponding to a mean summer irrigation) have to be applied per year, and if 60% of the available euent are to be reused, 6000 to 10 000 ha can be irrigated with the water discharged from these treatment plants. Nitrogen amounts will then widely exceed the needs of plant growth and may present risks for crops and/or groundwaters. Soil salinization risks have also to be taken into account (Bahri, 1987). Soils and groundwater pollution risks by trace elements are, on the contrary, not signi®cant.
ment plant, the proportion of industrial water compared to the domestic, and ®nally of the water supply quality. The water could be characterized by: . a variable organic load, sometimes exceeding the maximum values of the Tunisian standards related to euent quality for agricultural reuse purposes. The suspended solids content (<50 mg lÿ1) did not present, according to Ayers and Westcot (1985), potential for clogging problems in localized irrigation systems except for lagoon waters; . moderate salinity loads for most of the treatment plants except CoÃtieÁre Nord, Moknine, and Sfax where they were particularly high. This means, consequently, soil salinization risks. Alkalinization risks may not be important because of the high concentration of Ca and the elevated EC of the euents; . a much larger variability of organic parameters and nutrients compared to other parameters which in the case of N and P may be a constraint in using the water for fertilization purposes; . a high content in fertilizing elements but, however, nitrate pollution risks to the groundwater; . a low trace elements content, far below toxicity thresholds; . a salinity increase with time in some plants which may be related to climatic conditions and water management measures. The salt contents in wastewater were almost not aected by the treatment process. Removal eciency was around 48% for total N (NH4: 62%) and 63% for total P. For trace elements, removals averaged: Zn 87%, Cu and Fe 78%, Pb 64%, Cr 50%, Mn 44%, Co and Cd 17%, and Ni 13%. The removal eciency at a speci®c treatment plant depended on the type of treatment process. This survey on water quality of in¯uent and euent for treatment plants in Tunisia provided a comprehensive pro®le of the concentrations of fertilizers and pollutants that can be expected from reclaimed
CONCLUSION
The analysis of euents discharged from dierent wastewater treatment plants showed that their chemical composition varied from one treatment plant to another depending on the treatment processes employed, the seepage of brackish/sea water into the sewerage network, the location of the treat-
Fig. 2. Variation with time of the ionic composition of CoÃtieÁre Nord euents from 1988 to 1990.
Technical Note
water across the country. Other parameters, however, need to be studied such as trace organics, since future industrial connections to the sewerage system may aect the wastewater quality. REFERENCES
Akhter M. S. (1991) Chemical analysis of irrigation waters in Bahrein. Wat. Sci. Technol. 23, 2159±2164. Asano T., Smith R. G. and Tchobanoglous G. (1985) Municipal wastewater: treatment and reclaimed water characteristics. In Irrigation with Reclaimed Municipal Wastewater Ð A Guidance Manual, ed. G. S. Pettygrove and T. Asano, Ch. 2, pp. 1±26. Lewis Publishers, Chelsea, MI. Association Franc° aise de Normalisation (AFNOR) (1979) Recueil de Normes Franc° aises, Eaux Ð MeÂthodes d'Essais, 1st ed. Ayers R. S. and Westcot D. W. (1985) Water Quality for Agriculture, FAO Irrigation and Drainage Paper, No. 29, FAO, Rome, Italy. Bahri A. (1987) Utilization of treated wastewater and sewage sludge in agriculture in Tunisia. Desalin. 67, 233± 244. Brown H. G., Hensely C. P., McKinney J. L. and Robinson J. L. (1973) Eciency of heavy metal removal in municipal sewage plants. Environ. Lett. 5, 13±114. Chin K. K. and Ong S. L. (1991) A study of reclamation of sewage for industrial waters. Wat. Sci. Technol. 23, 2181±2187. Crook J., Ammerman D. K., Okun D. O. and Matthews R. L. (1992) Guidelines for Water Reuse. Camp Dresser and McKee Inc., Cambridge, MA, 254 p. Gras R. and Morisot A. (1973) Etude de l'eÂpandage des eaux reÂsiduaires des industries agricoles et alimentaires, MinisteÁre de l'Agriculture et du DeÂveloppement
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Rural, Direction des Industries Agricoles et Alimentaires, June. Gueddari M. (1984) GeÂochimie et thermodynamique des eÂvaporites continentales Ð Etude du Lac Natron en Tanzanie et du Chott El JeÂrid en Tunisie. State Doc. Diss., Louis Pasteur's University, Strasbourg, 143 p. Institut National de la Normalisation et de la ProprieÂte Industrielle (INNORPI) (1989) Environment protection: Use of reclaimed water for agricultural purposes Ð Physical, chemical and biological speci®cations (in French), Tunisian standards, NT 106.03. Lauer W. C. (1991) Water quality for potable reuse. Wat. Sci. Technol. 23, 2171±2180. Metcalf and Eddy (1991) Wastewater Engineering: Treatment, Disposal, Reuse. McGraw-Hill, New York, N.Y. Murphy J. and Riley J. P. (1962) A modi®ed single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27, 31±46. Nielsen J. S. and Hrudey S. E. (1983) Metal loadings and removal at a municipal activated sludge plant. Wat. Res. 17, 1041±1052. Oliver B. G. and Cosgrove E. G. (1975) Metal concentrations in sewage, euents and sludges of some southern Ontario wastewater treatment plants. Environ. Lett. 9, 75±90. Pettygrove G. S. and Asano T. (1985) Irrigation with Reclaimed Municipal Wastewater Ð A Guidance Manual. Lewis Publishers, Chelsea, MI. Piper A. M. (1944) A graphic procedure in the geochemical interpretation of water analyses. Trans. Am. Geophys. Union 35, 914±923. Rejeb S. (1990) Eects of wastewater and sewage sludge on the growth and chemical composition of some crop species (in French), 3rd Cycle Diss., Tunis, 164 p. Water Pollution Control Federation (1989) Water reuse, 2nd edn., Manual of practice SM-3, Water Pollution Control Federation, Alexandria, VA.
Wat. Res. Vol. 32, No. 11, pp. 3484±3489, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
TECHNICAL NOTE FERTILIZING VALUE AND POLLUTING LOAD OF RECLAIMED WATER IN TUNISIA M AKISSA BAHRI**
National Research Institute for Agricultural Engineering, Water, and Forestry, B.P. 10, Ariana 2080, Tunisia (First received September 1996; accepted March 1998) AbstractÐA national survey of wastewater composition was conducted for 28 chemical constituents from 15 treatment plants located across Tunisia to provide a basis for establishing water quality and management for crop production and environmental protection. The contents of salt, nutrients and trace elements were investigated and showed that the composition was characterized by a moderate salinity and sodicity for most of the plants, a high variability of the organic parameters, and a low trace elements content. In the case of N and P, such variations may be a constraint in using the water for fertilizing purposes. An increase in salinity with time was noticed for several plants. While the salt contents of wastewater were not aected by the treatment process, total N and P removal eciencies were 48% (62% for ammonium) and 63%, respectively. For trace elements, removals averaged: Zn 87%; Cu and Fe 78%; Pb 64%; Cr 50%; Mn 44%; Co and Cd 17%; and Ni 13%. The analyzed reclaimed water samples were consistent with the Tunisian standards regarding water quality required for agricultural reuse and had a high fertilizing content. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐwastewater quality, salinity, nutrients, trace elements, removal eciency
INTRODUCTION
MATERIALS AND METHODS
Physical, chemical, and biological parameters are important when characterizing wastewater for design of reuse facilities and in the management of environmental quality (Metcalf and Eddy, 1991). Municipal treatment plants usually have been designed to remove organic matter and suspended solids, but other wastewater constituents may be of concern for dierent reuse objectives (Asano et al., 1985) and thus have to be monitored.
The municipal wastewater was mainly domestic (80%) and processed through biological treatment plants up to the secondary level. The survey was done for the 15 following types of plants:
Many studies have been conducted on wastewater quality and its suitability for dierent purposes (Nielsen and Hrudey, 1983; Pettygrove and Asano, 1985; Akhter, 1991; Chin and Ong, 1991; Lauer, 1991; Metcalf and Eddy, 1991; Crook et al., 1992). In the Mediterranean area, such studies are rather few. In Tunisia, a survey over most of the sewage treatment plants supplying irrigation water was conducted to examine variation in water quality for individual treatment plant as well as between dierent treatment plants, removal eciencies, and to evaluate the fertilizing value and polluting load of municipal wastewater euents.
Besides these plants, reclaimed water used for irrigating an area (3200 ha) located 20 km north of Tunis which are Cherguia, CoÃtieÁre Nord, and Choutrana euents blended together has been analyzed. Whereas for some treatment plants (Cherguia, SE4, Choutrana, CoÃtieÁre Nord, Sud MeÂliane, and Sfax) the monitoring was done for several years, other plants and in¯uents were observed during much shorter periods. The sampling frequency was once a week for the plants located in the Tunis area; later, it was changed to once a month for all locations. Grab samples were taken from the stabilization ponds and composite samples (2 h) from the other treatment processes. Standard techniques were used to analyze the dierent parameters: pH, electrical conductivity (measured at 258C), total dissolved solids (TDS, evaporation at 1058C), suspended solids (SS) (®ltration on glass-®ber ®lter disk (Afnor, 1979)), chemical oxygen demand (COD) (potassium dichromate method (Afnor, 1979)), biochemical oxygen demand (BOD5) (incubation method (Afnor, 1979)), Na and K (¯ame photometric method), Ca and Mg
*[Tel: 216 1 719630, Fax: 216 1 717951].
. activated sludge: Cherguia, SE1, SE2, SE4, Sousse Nord, and Kairouan; . trickling ®lter: Monastir; . activated sludge + trickling ®lter: Sousse Sud; . oxidation ditch: Choutrana, Sud MeÂliane, and SE3; . stabilization ponds (SP) and aerated lagoons (AL): CoÃtieÁre Nord (SP), Moknine (SP), Gafsa (SP), and Sfax (AL).
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Technical Note (EDTA titrimetric method), Cl (argentometric method), SO4 (turbidimetric method), HCO3 (titrimetric method), Kjeldahl nitrogen (NK), NH4 (distillation or phenate method), NO3 (reduction through Cd±Hg column), NO2 (sulfanilamide diazotization), total phosphorus (P) (Na2S2O8±H2SO4 mineralization, Murphy and Riley (1962) colorimetric determination), phosphate ions (PO4) (spectrophotometric method (Afnor, 1979)), B (colorimetric method Ð azomethin H), trace elements (Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, and Zn) by atomic absorption spectroscopy (Perkin Elmer 2380 atomic absorption spectrophotometer) after concentrating 1 l of sample (with addition of HNO3). A geochemical characterization and statistical analysis were performed on the data.
3485
was less than the standard (30 mg lÿ1) in most of the treatment plants except in the lagoons, and in one trickling ®lter (Monastir). Minimum (1 mg lÿ1) and maximum values (357 mg lÿ1) were observed at Cherguia and Moknine treatment plants, respectively. The average value for the in¯uents was 360 mg lÿ1. Water discharged from the lagoons had much higher COD, BOD5, and SS content compared to the other processes because of algae development. For other plants, high values were due to overloading. Salt content
RESULTS AND DISCUSSION
The content of salts, fertilizers, and trace elements for wastewater samples is discussed in this paper. Data for euent from a given treatment plant (not shown) and for raw and reclaimed water from multiple plants (Tables 1 and 2) are reported. General characteristics Based on samples analyzed, the quality of euent was more or less consistent with the Tunisian standards for agricultural reuse (Table 2) (INNORPI, 1989). The pH of euent varied between 7.5 and 7.9 (with minimum value (6.8) observed at Monastir, SE3, and Sousse Nord, and maximum value (8.8) at CoÃtieÁre Nord). The average content of suspended solids was 42.5 mg lÿ1 (varying between 15 and 191 mg lÿ1). The maximum average value was found in Moknine (aerated lagoons). It
The minimum and maximum EC were respectively observed in Cherguia (1.16 dS mÿ1) and CoÃtieÁre Nord (12.63 dS mÿ1) plants. The average electrical conductivity of reclaimed water varied between 2.4 and 8.9 dS mÿ1 and that of the total dissolved solids was between 1.5 and 5.6 g lÿ1. In¯uents and euents had the same salinity range. Treatment processes did not aect the water salinity except for stabilization ponds in which salinity increased during warm periods. The in¯uent salinity was, in the case of CoÃtieÁre Nord, due to seawater seepage into the collection network, and at the plant location (near a salt lake). Sfax water supply, more mineralized than in the other cities, was partly responsible for the higher than normal salinity in its wastewater. For Moknine, the industrial activities (50% of the sewage water) and the plant location (in a salt lake), were both responsible for the sal-
Table 1. Descriptive statistics of average element concentration for in¯uents from 15 wastewater treatment plants in Tunisia (in mg lÿ1 unless otherwise indicated) Parameter
Mean
Median
SD
Min.
Max.
pH EC (dS mÿ1) TDS (g lÿ1) Ca Mg K Na HCO3 SO4 Cl SAR SS COD BOD5 Nk NH4-N NO3-N NO2-N P PO4-P Cd Co Cr Cu Fe Mn Ni Pb Zn
7.6 4.72 2.95 167 91 51 622 652 515 856 9.4 359.0 558.8 248.6 76.7 67.4 0.48 2.62 9.43 6.17 0.006 0.023 0.032 0.079 1.017 0.096 0.039 0.122 0.280
7.7 4.27 2.56 161 78 49 507 641 476 707 7.7 307.5 479.9 267.2 67.2 63.9 0.06 0.07 9.80 2.53 0.006 0.022 0.024 0.073 0.915 0.089 0.038 0.106 0.290
3.3 1.85 1.13 34 39 23 370 146 157 640 4.0 179.1 186.0 66.7 25.7 20.8 0.68 3.80 2.09 1.73 0.002 0.007 0.025 0.030 0.646 0.050 0.008 0.078 0.124
7.1 2.99 1.92 129 62 19 284 342 266 254 4.9 232.0 396.6 69.4 41.9 33.4 0.02 0.01 3.40 2.74 0.004 0.012 0.010 0.040 0.199 0.033 0.027 0.050 0.074
7.9 10.4 6.59 262 219 101 1790 888 840 2955 20.8 765.6 933.0 325.7 141.6 100.0 1.81 11.17 12.20 9.04 0.010 0.041 0.109 0.160 2.780 0.230 0.051 0.347 0.482
CV 0.033 0.392 0.383 0.205 0.429 0.455 0.596 0.224 0.305 0.746 0.431 0.499 0.333 0.268 0.335 0.308 1.425 1.452 0.222 0.280 0.333 0.304 0.781 0.380 0.635 0.521 0.205 0.639 0.443
SD: standard deviation; CV: coecient of variation; n: number of treatment plants; SAR: sodium adsorption ratio.
n 15 15 15 15 15 15 15 15 15 15 15 8 7 14 15 15 11 15 15 13 15 15 15 15 15 15 15 15 15
3486
Technical Note
Table 2. Descriptive statistics of average element concentration for euents from 15 wastewater treatment plants in Tunisia (in mg lÿ1 unless otherwise indicated) Parameter
Mean
Median
SD
Min.
Max.
pH EC (dS mÿ1) TDS (g lÿ1) Ca Mg K Na HCO3 SO4 Cl SAR SS COD BOD5 Nk NH4ÿN NO3-N NO2-N P PO4-P Cd Co Cr Cu Fe Mn Ni Pb Zn
7.6 4.10 2.61 168 85 52 537 524 532 791 8.5 42.5 173.6 35.3 30.0 26.2 9.5 2.48 3.60 2.34 0.005 0.019 0.016 0.017 0.226 0.054 0.034 0.044 0.036
7.6 3.42 2.23 171 72 44 431 473 484 625 7.2 20.0 103.2 26.8 26.5 23.7 7.2 1.80 3.30 1.90 0.005 0.016 0.016 0.016 0.181 0.048 0.033 0.042 0.033
0.1 1.68 1.08 31 36 27 293 189 147 526 3.8 47.9 152.7 18.4 11.0 10.6 6.0 2.19 1.63 1.08 0.001 0.006 0.004 0.004 0.115 0.025 0.009 0.008 0.011
7.5 2.39 1.52 121 54 18 293 333 304 338 5.1 14.7 61.4 17.8 16.9 14.4 2.1 0.52 1.61 1.23 0.004 0.012 0.009 0.011 0.108 0.022 0.021 0.035 0.023
7.9 8.94 5.61 238 188 120 1438 1046 922 2490 17.6 190.9 639.5 69.8 53.1 48.3 23.2 8.89 6.54 4.34 0.008 0.031 0.023 0.025 0.511 0.112 0.049 0.066 0.063
CV
NT
0.017 0.410 0.414 0.185 0.427 0.512 0.553 0.360 0.277 0.666 0.449 1.128 0.879 0.521 0.367 0.403 0.631 0.881 0.453 0.462 0.240 0.337 0.256 0.251 0.507 0.470 0.267 0.177 0.316
6.5±8.5 7.0 ÿ ÿ ÿ ÿ ÿ ÿ 2000 ÿ 30 (a) 90 (a) 30 (a) ÿ ÿ ÿ ÿ ÿ ÿ 0.01 0.1 0.1 0.5 5 0.5 0.2 1 5
SD: standard deviation; CV: coecient of variation; NT: Tunisian standards; (a): except special authorization. SAR: sodium adsorption ratio.
inity of the wastewater. The high salt content of wastewater at Sud MeÂliane may be the result of high density of industrial units. For both the raw and euent, Na and Cl were (in Piper (1944) diagram) the prevailing mineral ions except for wastewater from Kairouan and Gafsa in which HCO3 or SO4 was in greater concentration over Cl. Dierences between in¯uent and euent composition were essentially due to geochemical processes occuring during the treatment such as mineral precipitation (calcite). The SO4 concentrations were always higher than those of Ca. The chemical equilibrium model (SIMEQ) based on the Pitzer equations (speci®c ionic interaction model) (Gueddari, 1984) was used to test saturation index of some mineral salts. Almost all samples were over-saturated with respect to calcite except a few samples at Sousse Nord samples (with ionic strength less than 0.03 mol lÿ1) and under-saturated with respect to gypsum. The average value of the sodium adsorption ratio (SAR) of euents ranged between 8.7 and 17.6 at CoÃtieÁre Nord, Moknine, Sud MeÂliane, Sfax, SE1, and Monastir. The SAR of the other treatment plants varied between 5.1 and 7.4. The minimum average value was found for Gafsa and the maximum for CoÃtieÁre Nord.
n = 15) and varied generally between 27 mg lÿ1 for SE3 and 85 mg lÿ1 for Gafsa. Nitrogen was essentially present as ammonia except at SE2 where nitrate was prevailing. However, it was noted that in most of the plants and during certain periods, nitrates and sometimes nitrites were inversely related to ammonia in the euent (Fig. 1). In the in¯uent, the average total nitrogen content was 80 mg lÿ1 (CV = 0.35, n = 15) with a minimum value (44 mg lÿ1) for SE3, and maximum value (149 mg lÿ1) for Gafsa. The total nitrogen of the in¯uent consisted of essentially ammonia and small amounts of nitrate or nitrite. The average total phosphorus concentration in the euents was 3.6 mg lÿ1 (varying between 1.6
Nutrient content The average total nitrogen content (Norg+NH4+NO3+NO2) of the euent was about 42 mg lÿ1 (coecient of variation (CV) = 0.38,
Fig. 1. Variation with time of NH4 and NO3 concentrations in CoÃtieÁre Nord euents from 1988 to 1989.
Technical Note
and 6.5 mg lÿ1). Phosphorus was essentially in an inorganic soluble form. The average content of PO4 ions (PO4-P: 2.3 mg lÿ1) of euents varied from 1.2 mg lÿ1 for Choutrana to 4.3 mg lÿ1 for Gafsa. The average concentration in the in¯uents for total phosphorus was 9.4 mg lÿ1 with a range from 3.4 to 12.2 mg lÿ1. The average potassium content of the euents varied from 17.5 mg lÿ1 for SE3 to 120 mg lÿ1 for Moknine. The average for all plants was 52 mg lÿ1. The same average value was found in the in¯uents. A comparison of the fertilizing content in the euent showed that, in all the water samples, the amount of phosphorus was much less than that for nitrogen and potassium: N=P2 O5 =K2 O:1=0:2=1:6
N=P=K:1=0:1=1:3:
Trace element content Trace element concentrations of in¯uents and euents were all below the maximum concentrations recommended for agricultural reuse by the Tunisian standards (Table 2) (INNORPI, 1989). In the euents, the highest concentration for most of the trace elements was found for Sfax and the lowest for SE3. The following sequences were common in the in¯uent and euent: influent: Fe Zn > Pb > Mn > Cu > Ni > Cr > Co > Cd effluent: Fe Mn > Pb > Zn > Ni > Cu > Co > Cr > Cd Average boron content was around 0.8 2 0.2 mg lÿ1 in the in¯uent (ranging from 0.3 to 1.7 mg lÿ1) and 0.7 2 0.2 mg lÿ1 in the euent (ranging from 0.3 to 1.4 mg lÿ1). These values were within the usual range in irrigation water (Ayers and Westcot, 1985). Statistical characteristics Descriptive statistics of element concentration in raw and reclaimed water are given in Tables 1 and 2. The data for euent samples obtained from dierent treatment plants (not shown) showed that the extent of temporal variability in composition of euent depended on parameters. Parameters relating to organic matter and trace element content in the water exhibited a considerable variability (CV = 0.40±1.83 on average) with nitrite and nitrate displaying the extreme variability (CV up to 2.50). The CV was lower for the major ions, EC, and TDS (CV < 0.25 on average). The variability of water quality between dierent treatment plants (Tables 1 and 2) was larger for the organic matterrelated parameters compared to the inorganic ones (major and minor elements). The CV for the trace
3487
elements of reclaimed water from dierent treatment plants was smaller than that for a given treatment plant and the contrary was the case for the major ions due to the dierent salt contents of the wastewater. The variation of trace element between treatment plants and especially for euents was then within a relatively narrow range (CV about 0.30 except for Fe and Mn). The Kolmogorov±Smirnov test was used to ®t theoretical probability distribution functions (pdf) to the dierent parameters in the euent. Theoretical normal and/or lognormal pdfs could, in several cases, be ®tted to the parameters except Cd. The suspended solids, COD, BOD5, NO3, and NO2 had more skew distributions that followed lognormal distributions. However, as the number of data increased, as in the case of Cherguia and SE4, the parameters showed poor accordance with the normal or lognormal pdfs. This may be due to the presence of several outliers creating discontinuities in the empirical pdfs. The variability in the euent quality may also reveal some inherent in-plant treatment problems: The Cherguia plant was renovated at the end of the study period. The minimum number of samples needed for mean value estimation with a relative error margin of 10% at a probability level of 0.05 was calculated for the dierent parameters and treatment plants. It was found that this number varied widely from 3± 30 for parameters related to the salt content (EC, TDS, Ca, Mg, Na, K, Cl, HCO3, and SO4), 6±66 for SS, COD, and BOD5, 5±300 for trace elements, and 17±>500 for the dierent nitrogen and phosphorus forms. Process removal eciency A comparison of average sewage in¯uent and euent composition was made to investigate the overall removal eciency. The salt contents of wastewater were not aected by the treatment process as expected. Average COD and BOD5 removal eciencies were 69 and 86%, respectively, while that of total N was 48% (62% for NH4) and that of total P was 63%. For trace elements, the most signi®cant removal was found for Zn (87%), followed by Cu and Fe (78%), Pb (64%), Cr (50%), Mn (44%), Co and Cd (17%), and Ni (13%). These removal eciencies are similar to that found by WPCF (1989) for some secondary treatment processes. Other removal eciencies can be found in the literature (Brown et al., 1973; Oliver and Cosgrove, 1975; Nielsen and Hrudey, 1983) depending on treatment process, loading rate, and in¯uent quality. In this study, Zn, Cu, Fe, and Pb can be assumed to have high contents in the sewage sludge. Substantial amounts of C, N, and P should also be expected in the sewage sludge. On the other hand, the removal eciency depended on the type of treatment process. For example, activated sludge, stabilization ponds, and
3488
Technical Note
trickling ®lter were more ecient in Nk removal (60±70%) compared to the oxidation ditch (51%). For P, activated sludge and oxidation ditch had higher removal eciencies compared to stabilization ponds. For Cu, Fe, Pb and Zn, higher removal eciencies were obtained with the oxidation ditch compared to the activated sludge process or to the stabilization ponds. Quality changes versus time The quality changes with time for the most important treatment plants (Cherguia, Choutrana, CoÃtieÁre Nord, Sud MeÂliane, and SE4) were evaluated. The composition of euent at a treatment plant varied with time (increase of salinity in Cherguia and Choutrana euents), depending on the eciency of the treatment plant, the proportion of water produced by the dierent activities, etc. Seasonal variations were observed in the CoÃtieÁre Nord water due to long detention time in the lagoons (Fig. 2), on the one hand, and salt reduction from 1988 to 1992 due to the prevention of seawater intrusion into the collection network, on the other hand. Fertilizing value and polluting load of reclaimed water The impacts of euent application on the soil± plant±groundwater system have to be evaluated. In previous studies, in Tunisia, it was demonstrated that the fertilizing units brought by euent had a favourable eect on the growth of certain crops and on their N, P, and K content (Rejeb, 1990). If the N, P, and K average crop uptakes are assumed to be 80 kg N, 45 kg P, and 85 kg K per hectare (Gras and Morisot, 1973), the amount of fertilizing elements present in the monitored wastewater plants may cover the N needs of about 50 000 ha, the P of about 8000 ha, and the K of 60 000 ha. If, at the other hand, 600 to 1000 mm (value corresponding to a mean summer irrigation) have to be applied per year, and if 60% of the available euent are to be reused, 6000 to 10 000 ha can be irrigated with the water discharged from these treatment plants. Nitrogen amounts will then widely exceed the needs of plant growth and may present risks for crops and/or groundwaters. Soil salinization risks have also to be taken into account (Bahri, 1987). Soils and groundwater pollution risks by trace elements are, on the contrary, not signi®cant.
ment plant, the proportion of industrial water compared to the domestic, and ®nally of the water supply quality. The water could be characterized by: . a variable organic load, sometimes exceeding the maximum values of the Tunisian standards related to euent quality for agricultural reuse purposes. The suspended solids content (<50 mg lÿ1) did not present, according to Ayers and Westcot (1985), potential for clogging problems in localized irrigation systems except for lagoon waters; . moderate salinity loads for most of the treatment plants except CoÃtieÁre Nord, Moknine, and Sfax where they were particularly high. This means, consequently, soil salinization risks. Alkalinization risks may not be important because of the high concentration of Ca and the elevated EC of the euents; . a much larger variability of organic parameters and nutrients compared to other parameters which in the case of N and P may be a constraint in using the water for fertilization purposes; . a high content in fertilizing elements but, however, nitrate pollution risks to the groundwater; . a low trace elements content, far below toxicity thresholds; . a salinity increase with time in some plants which may be related to climatic conditions and water management measures. The salt contents in wastewater were almost not aected by the treatment process. Removal eciency was around 48% for total N (NH4: 62%) and 63% for total P. For trace elements, removals averaged: Zn 87%, Cu and Fe 78%, Pb 64%, Cr 50%, Mn 44%, Co and Cd 17%, and Ni 13%. The removal eciency at a speci®c treatment plant depended on the type of treatment process. This survey on water quality of in¯uent and euent for treatment plants in Tunisia provided a comprehensive pro®le of the concentrations of fertilizers and pollutants that can be expected from reclaimed
CONCLUSION
The analysis of euents discharged from dierent wastewater treatment plants showed that their chemical composition varied from one treatment plant to another depending on the treatment processes employed, the seepage of brackish/sea water into the sewerage network, the location of the treat-
Fig. 2. Variation with time of the ionic composition of CoÃtieÁre Nord euents from 1988 to 1990.
Technical Note
water across the country. Other parameters, however, need to be studied such as trace organics, since future industrial connections to the sewerage system may aect the wastewater quality. REFERENCES
Akhter M. S. (1991) Chemical analysis of irrigation waters in Bahrein. Wat. Sci. Technol. 23, 2159±2164. Asano T., Smith R. G. and Tchobanoglous G. (1985) Municipal wastewater: treatment and reclaimed water characteristics. In Irrigation with Reclaimed Municipal Wastewater Ð A Guidance Manual, ed. G. S. Pettygrove and T. Asano, Ch. 2, pp. 1±26. Lewis Publishers, Chelsea, MI. Association Franc° aise de Normalisation (AFNOR) (1979) Recueil de Normes Franc° aises, Eaux Ð MeÂthodes d'Essais, 1st ed. Ayers R. S. and Westcot D. W. (1985) Water Quality for Agriculture, FAO Irrigation and Drainage Paper, No. 29, FAO, Rome, Italy. Bahri A. (1987) Utilization of treated wastewater and sewage sludge in agriculture in Tunisia. Desalin. 67, 233± 244. Brown H. G., Hensely C. P., McKinney J. L. and Robinson J. L. (1973) Eciency of heavy metal removal in municipal sewage plants. Environ. Lett. 5, 13±114. Chin K. K. and Ong S. L. (1991) A study of reclamation of sewage for industrial waters. Wat. Sci. Technol. 23, 2181±2187. Crook J., Ammerman D. K., Okun D. O. and Matthews R. L. (1992) Guidelines for Water Reuse. Camp Dresser and McKee Inc., Cambridge, MA, 254 p. Gras R. and Morisot A. (1973) Etude de l'eÂpandage des eaux reÂsiduaires des industries agricoles et alimentaires, MinisteÁre de l'Agriculture et du DeÂveloppement
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Rural, Direction des Industries Agricoles et Alimentaires, June. Gueddari M. (1984) GeÂochimie et thermodynamique des eÂvaporites continentales Ð Etude du Lac Natron en Tanzanie et du Chott El JeÂrid en Tunisie. State Doc. Diss., Louis Pasteur's University, Strasbourg, 143 p. Institut National de la Normalisation et de la ProprieÂte Industrielle (INNORPI) (1989) Environment protection: Use of reclaimed water for agricultural purposes Ð Physical, chemical and biological speci®cations (in French), Tunisian standards, NT 106.03. Lauer W. C. (1991) Water quality for potable reuse. Wat. Sci. Technol. 23, 2171±2180. Metcalf and Eddy (1991) Wastewater Engineering: Treatment, Disposal, Reuse. McGraw-Hill, New York, N.Y. Murphy J. and Riley J. P. (1962) A modi®ed single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27, 31±46. Nielsen J. S. and Hrudey S. E. (1983) Metal loadings and removal at a municipal activated sludge plant. Wat. Res. 17, 1041±1052. Oliver B. G. and Cosgrove E. G. (1975) Metal concentrations in sewage, euents and sludges of some southern Ontario wastewater treatment plants. Environ. Lett. 9, 75±90. Pettygrove G. S. and Asano T. (1985) Irrigation with Reclaimed Municipal Wastewater Ð A Guidance Manual. Lewis Publishers, Chelsea, MI. Piper A. M. (1944) A graphic procedure in the geochemical interpretation of water analyses. Trans. Am. Geophys. Union 35, 914±923. Rejeb S. (1990) Eects of wastewater and sewage sludge on the growth and chemical composition of some crop species (in French), 3rd Cycle Diss., Tunis, 164 p. Water Pollution Control Federation (1989) Water reuse, 2nd edn., Manual of practice SM-3, Water Pollution Control Federation, Alexandria, VA.