INTRODUCTION:

Sludge is a voluminous waste, generally hydrated to 95% and 99%; it is essentially composed of water, suspended mineral matter, non-biodegradable organic matter and microorganisms; it is primarily made up of fermentable organic matter and is, therefore, a potential source of nuisance^1–3. High nutrients are present in the sludge produced by wastewater treatment facilities, which is further distinguished by high heating values. As a result, the sludge that has been given an organic matter and nutritional boost is a possibility for use as fertilizer and a feedstock for alternative energy sources.

The need for alternative energy sources is now urgent since energy independence helps to maintain the nation’s economic stability^4–8. In Morocco, several wastewater treatments plants (WWTP) have been set up in the last few years because of demographic growth, human, industrial and agricultural consumption and repeated droughts. The wastewater treatment plant of the city of Kenitra, which will be commissioned in 2020, covers an area of 12 ha and is one of the most critical plants in Morocco (Figure 1). It treats large quantities of wastewater, 19.6million/year. At the exit of WWTP-Kenitra, the purified water, thrown in the natural environment (OuedSebou), is accompanied by a production of significant quantities of sludge. An average of 27 tons per day of dewatered sludge is produced by the plant and deposited in the city’s landfill. Exposing waste sludge is a real challenge for wastewater treatment plant managers. Indeed, sludge production is increasing yearly due to population growth and regulations that are increasingly strict and demanding in terms of wastewater discharge standards. Landfilling is a poor technique and is legally prohibited in many countries. The valorization of sewage sludge is often limited by harmful elements, such as toxic chemical substances (heavy metals), or more simply by the harmful odours of sludge since it can be highly fermentable in the environment^9–14. Indeed, WWTP sludge contains, on the one hand, exciting constituents for fertilization, i.e. organic matter and mineral elements such as phosphorus and nitrogen. On the other hand, harmful components whose concentration in the sludge must be measured to ensure that it is below the norms; this is particularly important for agricultural valorization^15–17.

The objective of our work is the physico-chemical parameters control of the dehydrated sludge produced by the Kenitra sewage treatment plant in the aim of a valorization in the agricultural field.

Figure 1: The wastewater treatment plant of Kenitra city

MATERIALS AND METHODS:

Study area and sampling:

Each wastewater treatment plant has two treatment lines, one called the “water line,” and the other called the “sludge line”. The sludge chain is different from one region of the world to another and from one plant to another¹⁸. The independent company of Kenitra “RAK”, responsible for the WWTP of Kenitra, adopts the vision of valorization of the waste sludge in the field of agriculture, if it meets the requirements. In this context, we undertook to conduct Physico-chemical analyses of dehydrated sludge. Samples of dewatered sludge from an intensive system (activated sludge) were taken regularly for three months based on the Moroccan standard and analyzed in two different laboratories: an internal laboratory and an external laboratory, to verify the results obtained. At the Kenitra WWTP, sludge sampling is done directly on the dewatered sludge discharge site at the outlet of the belt filter treatment in a plastic box.

Hydrogen potential analysis (pH):

According to French standard AFNOR NF-T 90-015¹⁹, 1 g of dehydrated sludge diluted in 10ml of distilled water, shake for 15min and let the solution settle for 4h. Clean the probe membrane with distilled water after each use and dab lightly with toilet paper. Finally, calibrate the electrode with the buffer solutions, immerse the electrode in the solution “the supernatant” to be measured, and read the results directly on the screen.

Dryness:

The dryness is the percentage of dry matter in the fresh sample. Weigh an empty aluminum cup, and the weight P1 is obtained. Then place a quantity of sample (2 to 5g) in the cup, the weight P2 got. Place the cup in the oven at 105°C overnight. The next day, put the sample in the desiccator for half an hour. Finally, weigh again, and the weight P3 is obtained. The following formula allows us to calculate the value of the dryness S:

P3 – P1

S= ------------------------ X 100

P2 – P1

Dry matter (DM) and volatile dry matter (VDM):

The dry matter is calculated for a sample that cannot be filtered because of its load. It gives an idea about the proportion of solid matter in a liquid sample. Weigh aluminum cups without any bag; the weight P1 has been obtained. Place a volume V of the sample in each cup. Then place the cups in the oven at 105°C overnight. The next day, put the samples in the desiccator for half an hour. Then weigh them, and the weight P2 obtained. The following formula allows us to calculate the MS value in g/L.

P2 – P1

MS= ------------------------ X 1000

To determine the volatile dry matter, the following WWTPs are followed:

· Place the same samples used to determine the dry weight in the muffle oven at 550°C, after 2h, at a temperature of 105°C.

· Remove the pieces and place them in the desiccator.

· Measuretheir P3 weight.

The following formula is used to calculate the value of SVD in g/L:

P2 – P3

MVS= ------------------------ X 1000

Total Kjeldahl Nitrogen (TKN):

The determination of NTK is done in three WWTPs: mineralization, distillation, and determination; 1) Mineralization which consists in introducing 5g of sludge in Kjeldahl matras with 50ml of distilled water, then adding a mixture of some glass beads to regulate boiling, 5g of catalyst and 10ml of concentrated sulfuric acid. Place the matras in the mineralization block covered by the fume extraction system and connect the extraction system. Finally, bring the mixture to boil until evaporation and the appearance of white fumes. The dosage has been forced for 2 hours; the residual liquid must be clear. 2) The distillation consists in placing the Kjeldahl matras on the steam drive system, then adding 50ml of sodium hydroxide NaOH at 400g/L. To collect the distillate, place at the outlet of the apparatus a 250 ml Erlenmeyer flask containing 50ml of boric acid at 10 g/L, and admit steam for about 20minutes. 3) The determination consists of titrating the collected distillate with chloridric acid HCL at 0.01mol/L until the purple coloration turns and notes the volume used. A blank determination is done in parallel in the same way but using about 100ml of water instead of the test sample. The concentration of Kjeldahl nitrogen or ammoniacal nitrogen, expressed in mg/L of nitrogen (N), is calculated by the formula:

(V1 – V0) X 1000 X C x 14

[N] = ------------------------------------------

C= concentration (in mol/l) of the sulfuric acid solution used for the determination.

V1 = volume (in ml) of sulfuric acid used for the determination of the sample.

V0 = volume (in ml) of sulfuric acid used for the determination of the blank.

V = volume (in ml) of the test sample; 14 is the relative atomic mass of Nitrogen.

Total Phosphorus:

The determination of total phosphorus is done in two WWTPs: 1) The extraction consists of adding 2 ml of nitric acid and 2,5ml of perchloride acid and some pieces of beads to Kjeldahl matras containing 1g of dehydrated sludge. Then place the matras in the mineralization block, covered by the fume extraction system, connect the extraction system and start the mineralization apparatus (relative temperature graduation close to 6°C). Allow to cool after 2 hours of heating, and add 20ml of distilled water in the matras, then filter and recover in a flask. Complete the filtrate obtained with distilled water up to 100 mL (Solution to be determined). 2) Assay by adding 0.4 ml of sulfuric acid, 1ml of ascorbic acid and 1 ml of sodium thiosulfate to 40ml of the solution to be assayed and leave to stand for 10 minutes, then add 2ml of the ammonium molybdate solution and make up to 50ml with distilled water The solution is determined spectrophotometrically at 880nm. A control solution (blank) was prepared in parallel containing all the reagents without a sludge sample. The expression of the results consists in drawing a calibration curve using the solutions with known concentrations; the concentration of our solution to be determined can be concluded.

Potassium:

Add 100ml of ammonium acetic acid (1N) to 4g of dehydrated sludge. Then stir for 1 hour and filter the mixture into a 100mL flask. Finally, determine the filtrate by flame emission spectrometry. The potassium concentration is concluded from the calibration curve.

Minerals: Calcium and magnesium:

Measure the weight of the empty crucible and add 10g of dehydrated sludge in each crucible, then put the crucibles in the oven for 3h at 105°C and then in an oven at 450°C/1h. Add to 2g of dry matter obtained 10ml of HCl 25% and 5ml of the buffer solution for the total determination, then supplemented to 50 ml with distilled water and filter. In case of calcium dosage only, add 5 ml of soda. Add a spatula of Eriochrome-black dye to the filtrate for the total determination (Mg+Ca) and a spatula of HSN dye for the calcium determination. Finally, measure with EDTA (ethylene diamine tetra-acetic acid) until the colour changes.

Heavy metals:

Figure 2 shows the different steps for the preparation of a sludge extract for analysis and determination of heavy metal content.

The extract of metals is then simultaneously determined by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Different heavy metals have determined by this method.

Nutrient and heavy metal analysis was conducted in three trials each day during the month of April, the results presented in the results section represent the averages for each week

Figure 2: Protocol for the preparation of sludge extracts

RESULTS AND DISCUSSIONS:

The curve in the figure 3 describes the variation in the pH of the dewatered sludge. It varies between a minimum of 7,45 and a maximum of 8,7. For our samples, the pH average is 8,11. So these sludges are relatively alkaline. Generally, the pH of the sludge is located in the same range of values: 7 < pH < 8²⁰.

Figure 3: pH values of the dewatered sludge of the WWTP during March and April

The study conducted by⁴ shows that the pH of sludge from three different regions was shifting towards neutrality, as well as different other researches show that the pH values are between 6 and 8, such as the study of ²¹^,²²^,²³ and ²⁴ which found pH values of 6.3, 7.1, 7, 26 and 7.9 respectively. On the other hand, sludgecanbevery basic, with a pH of 12²⁵. In our case, the pH found is weakly alkaline (8.11) due to the addition of lime during dewatering, which increases the pH. This sludge can positively impact acidic soils to avoid the increase of heavy metals’ mobility and their absorption by plants.

Figure 4: Dryness measurement in the sludge of the WWTP during March and April

According to the curve in (Figure 4), the dryness of the sludge varies between a maximum of 26.92% and a minimum of 19%. The average being 22.36% is acceptable for a requirement of 20±2% for belt filter dewatering. A slurry's physical state is influenced mainly by its dryness, which stands for its dry matter content. Establishing precise dryness limit values for each physical state might be challenging because of the diversity of the produced sludge. According to²⁶, the physical state of residual sludge as a function of dryness is divided as follows; dryness ˂10% represents the liquid state; dryness between 10 and 25% represents a pasty state; dryness between 25and 50% represents a solid state and finally if the dryness is greater than 50%, we say that the state is granular. According to our results of dryness with an average of 22.36%, we can note that the sludge is in a pasty state, which represents an ideal state for the valorisation in agriculture and land application²⁷. Contrary to our results, another study showed that the dryness can reach 70% with a minimum percentage of 44%, which shows that the sludge can be in a solid or granular state²³.

Figure 5: Measurement of dry matter (DM) and volatile dry matter (VDM) of the WWTP sludge during March and April

The dry matter represents the suspended mineral and organic part, as well as the dissolved salts of the sludge. Therefore, the DM concentration is always higher than the VDM concentration because the latter represents the biodegradable fraction of the dry matter. From Figure 5, the average DM is 31.11g/l, while the average VDM is 16.77g/l. The dry matter value varies from one type of sludge to another, and from one season to another; for septage sludge, DM reaches 32% in summer, and 23% in winter²⁸. According to the results found at the WWTP, VDM presents 53% of dry matter, so the remaining 46% presents the mineral part. Comparing with the data conducted by²⁹, it can be concluded that the percentages of DM and VDM respect the ranges generally found in sludge. The MSV is composed of organic fertilizing elements such as carbon, nitrogen and phosphorus. For this reason, it was interesting to analyze the concentrations of some elements.

Table 1: Results of four weeks sludge analysis of fertilizing elements

Parameters	Unit	Date of sampling
Parameters	Unit	Week One	Week two	Week three	Week four
TKN	mg/Kg	84,1	22,4	33	44,8
Total Phosphorus	mg/Kg	680	992	705	555
Potassium	mg/Kg	130	125	322	62
Calcium	mg/Kg	47	67	31	62
Magnesium	mg/Kg	462	369	642	778

Table 2: Results of four weeks sludge analysis of heavy metals

Parameters	Unit	Date of sampling				Standard (Decree of 08/01/98) ³⁵
Parameters	Unit	Week One	Week two	Week three	Week four	Standard (Decree of 08/01/98) ³⁵
Zn	mg/Kg	391	371	310	423	3000
Cd	mt/Kg	0,22	0,25	0,21	0,21	20 (1)
Cr	mg/Kg	7,85	42,90	18,1	356	1000
Cu	mg/Kg	33,50	35,20	32,9	48,3	1000
Mn	mg/Kg	48,00	45,0	44,20	47,80	-
Ni	mg/Kg	7,40	7,77	6,81	8,65	200
Pb	mg/Kg	11,20	13,60	10,20	16,0	800
Hg	mg/Kg	1,15	1,44	1,11	1,63	10
Cr+Cu+Ni+Zn	mg/Kg	439,75	456,87	367,81	835,85	4000

During four weeks of monitoring and analysis of sludge, it was noticed that these sludge samples are rich in fertilizing elements such as TKN with important contents that reaches 84.1 mg/ kg, total phosphorus that varies between 555mg/kg and 992mg/kg, Potassium that exceeds 300mg/kg, and Magnesium that exceeds 770 mg/ Kg, on the other hand Calcium is detected with low contents (Table 1). The values obtained in our study are significant and higher than other values such as 4.35%± 0.15, 1.41%±0.01 obtained respectively by³⁰ and³¹. The rate of Total Phosphorus obtained by³² is between 328 and 1485 mg/Kg, which shows that the sludge analyzed in our work is very rich in Total phosphorus which important for plant growth. The values of Calcium, although they are low, but they are higher than other values (38mg/Kg - 35.6mg/Kg) obtained respectively by ³³ and ²⁵, and lower than 111.9mg/Kg³⁴. Generally, these sewage sludges have a real agronomic interest for soils and plants, given their richness in fertilizing elements.

The results concerning the determination of heavy metals in the dewatered sludge are grouped in table 2, they are compared to the Decree of 08/01/98 fixing the technical requirements applicable to the spreading of sludge on agricultural soils taken in application of the decree n° 97-1133 of 08/12/97 relating to the spreading of sludge from the treatment of waste water. The values of heavy metals in the analyzed sludge samples are very low compared to the limit values described in the Decree of 08/01/98 of the French Republic. Comparing our results with the results of other studies, we can conclude that the dewatered sludge in our case does not cause any toxicity risk for the soil and the environment due to the low quantities of heavy metals. Table 3 summarizes the results obtained in several studies in comparison with the highest values determined in our study. Finally, the heavy metals analyzed in the dewatered sludge meet the requirements for land application and do not exceed the regulatory thresholds.

Table 3: Comparison of heavy metal analysis results from our work with another research

Parameters	Unit	Our results	³⁰	³⁶	²⁴	²⁵
Zn	mg/Kg	423	624	470	731	500
Cd	mt/Kg	0,25	0,82	0,4	3	5
Cr	mg/Kg	42,9	25,4	30	52	115
Cu	mg/Kg	48,3	127,4	270	205	230
Ni	mg/Kg	8,65	27,6	20	25	35
Pb	mg/Kg	16,0	81,4	20	80,5	69
Cr+Cu+Ni+Zn	mg/Kg	835,85	804,4	750	1013	880

We note that the average of our results is within the norm; comparing them with other studies, we note that the sludge from the Kenitra wastewater treatment plant does not contain a very high level of heavy metals such as Cadmium, Zinc, Copper, Nickel and Lead.

CONCLUSION:

The results of this study showed that the treated sludge of the WWTP of the city of Kenitra, are rich in nutrients such as TKN which exceeds 84 mg/Kg, Total Phosphorus which reaches 992 mg/Kg, and Potassium with a value of 322 mg/Kg. The concentration of toxic trace metals such as Zn, Cd, Cr, Cu, Mn, Ni, Pb and Hg do not exceed the limits described in the EU standards. Therefore, the results of the analysis of the dewatered sludge in this project are compatible with the regulations for agricultural valorization in terms of physicochemical parameters.

The agricultural valorization of waste sludge can be considered as the most suitable recycling method to rebalance the biogeochemical cycles (N, K, P...) and to ensure the protection of the environment. Sludge is of great economic interest if it respects the requirements of land application and does not exceed the regulatory thresholds, because it contains most of the elements present in synthetic fertilizers, and therefore constitutes a highly appreciable nutritional reservoir for agronomic use, if it respects the requirements of land application and does not exceed the regulatory thresholds.

Microbiological characterization of dewatered sludge is recommended, with the objective of determining the sanitary status of the sludge before its use in the environment.

CONFLICT OF INTEREST:

All the authors declare that there is no conflict of interest.

ACKNOWLEDGEMENT:

Thanks to the research team in the WWPT of Kenitra.

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Received on 24.03.2023 Modified on 28.04.2023

Research J. Pharm. and Tech 2023; 16(11):5366-5371.

DOI: 10.52711/0974-360X.2023.00869