Dewatering, Stabilization, and Final Disposal of Waste Activated Sludge in Constructed Wetlands // Deshidratación, estabilización y disposición final de descarte de lodos activados en humedales construidos

Objective: This research aimed to assay the dehydration and stabilization of surplus biosolids from a wastewater activated sludge treatment plant, with sludge-treatment wetlands, at the Iguazú National Park (Misiones, Argentina). Materials and Methods: A 12-cell sludge-treatment reed beds (STRB) system was built and operated for four years. Afterwards, the accumulated sediments were analyzed for total solids (TS) concentration, volatile solids (VS) reduction, specific oxygen-uptake rate (SOUR), and heavy metals and pathogens concentrations. Results and Discussion: TS concentration increased from 0.55 % to 14.3 %, VS were reduced by 33.3 %, and SOUR lowered to 1.09 mg O2gTS-1 h-1. These figures and the final concentrations of heavy metals and pathogenic microorganisms (102 MPN E.coli.gTS-1) indicated a degree of stabilization and sanitation that allowed classifying these biosolids as Class A, according to the Argentine guidelines, National Resolution 410/18, and enabled their reuse for soil amendment in landscapes and other agronomic purposes. Conclusion: The technology tested showed good results applied under a tropical climate, with annual temperatures between 17 and 27 °C, rainfall of 1870 mm y-1, and planted with autochthonous vegetation. Furthermore, it allowed the reuse of 221 t (144 m3) of a harmless product in an environmentally sustainable way.


Introduction
Biological wastewater treatment systems, especially those using a suspended biomass like activated sludge, generate a surplus of microbial biomass (sludge, biosolids) that must be systematically discarded in order to ensure the stability of the treatment process and the quality of the final effluent. These residues need further treatment and a disposal that involves the dehydration of the mud, the reduction of the organic load, removal of pathogens, and the control of heavy-metal concentrations -procedures that represent a problem worldwide.
A technology designed for the treatment of waste biosolids in constructed wetlands referred to as the sludge treatment in reed beds, STRB by Nielsen and Bruun [1], the planted drying beds, PDB by Kengne and Tilley [2], and the sludge-treatment wetlands, STW by Uggetti et al. [3] -has proven to be a suitable alternative to the dehydration and stabilization of surplus sludge both environmentally and economically. These systems are made up of 8 to 12 cells with a filtering floor that is planted with helophytes. Every day, waste sludge is poured into each cell. The water content filters through the floor and is thus returned to the reactor, while the solid wastes are retained on the surface. The physiology of the plants favors further sludge dehydration through water absorption and transpiration. The microbial activity within the root environment (rhizosphere) digests and stabilizes the organic fraction of the sludge. When the capacity of the cells is reached (after 5 to 10 years), the accumulated residue is removed, and the filtering surface is recomposed and replanted. The cells then become operable again. If the treated residue meets certain quality standards, that material can be used as fertilizer or for soil amendments or other defined agronomic purposes.
The Iguazú National Park (INP, Misiones, Argentina, 25º 40' 40'' S, 54º 26' 42'' W), has a secondary-level treatment plant (activated sludge) to treat the wastewater generated in the sanitary and gastronomic services of the park. In 2012, a 12-cell wetland system was designed for the treatment of surplus biosolids, utilizing the filtering floor and drainage from the existing sludge-drying beds. After four years of operation, the sediments were sampled and analyzed as required by the National Resolution 410/18 [4], to classify the treated biosolids and to define their possible utilization or final disposal destination. This is our first experience in the application of this technology in Argentina. The aim of the project was first to analyze the performance of the wetland system under local climatic conditions and with native vegetation, and then to evaluate if the sludge so treated met the requirements of the current regulations for further use for agronomic purposes.

Experimental Set-Up
Climate. The Iguazú climate is defined as humid subtropical, with an average medium temperature of 22 °C (range 17 to 27 °C) and an average rainfall of 1,870 mm yr -1 (monthly average 156 mm; range 94 to 226 mm).  Then a submerged pump transfers every day the concentrated sludge (applied sludge) to the next cell following the numerical order. When all the sludge has been transferred, the clarified supernatant, water with some remaining solids, is returned from the thickener to the aeration basin (returned solids).

Sampling
Sludge sampling. Every day a settling test was performed on a ML sample and recorded as the SV30 (mL/L -1 ), the solids volume accumulated after a 30-min settling in a 1 L cylinder. On 35 sampling dates, 6 to 10 each year randomly distributed during the operational period, the amount of total (TSS) and volatile (VSS) suspended solids in the waste sludge, were also determined. For each sample, the sludge volume index (SVI, mL/g -1 ) was calculated as the SV30 divided by the ML TSS (g/L -1 ) after Metcalf & Eddy [5]. This database was used to further estimate the concentrations of the waste-sludge solids per day, as the SV30 divided by the average SVI. In a complementary fashion, on eleven sampling dates, the applied sludge and the clarified supernatant water were sampled to determine the concentration of the returned solids.
Final residue sampling. After four years of operation, the odd-numbered cells, originally a bit shallower, were almost full. These 6 cells were inactivated and left to rest for 5 months in order to enable the dewatering and stabilization of the residue upper layers. Afterwards, three samples of the accumulated material (60 cm depth) were taken from each cell through a center line, at both ends and, in the middle, with a 110-mm-diameter pipe used as a core sampler. The three samples from each cell were mixed to give the composite samples C1, C3, C5, C7, C9, and C11, from the correspondingly numbered cells. The reduction in volatile solids (VS) the specific oxygen-uptake rate (SOUR), and the Escherichia coli concentrations were measured in each composite sample. The Salmonella sp. and the viable-helminth-egg concentrations were measured on three samples composed of C1 + C3, C5 + C7, and C9 + C11 (i.e., 3 combined samples). For the heavy-metal-concentration analyses, the samples were grouped into C1 + C3 + C11 and C5 + C7 + C9 (i.e., 2 combined samples).

Analysis of Samples
The TSS of the waste sludge and applied sludge were determined on samples filtered through weighed standard glass-fiber filters and dried to constant weight at 105 °C (APHA, 2540 D; 6). The VSS were calculated as the weight lost from the same filters after ignition at 550 °C (APHA 2540 E; 6). Total solids (TS) and volatile solids (VS) of the residues in the cells were determined by the same methods that were applied to residue samples (APHA 2540 G; 6). To evaluate the vector-attraction potential, VS reduction was determined as the difference between the VS concentration in the sludge applied to the cells and that in the residue of the cell and Specific Oxygen Uptake Rate (SOUR) was determined by dilution of a weighed quantity of residue sample containing 1.5 g TS in a convenient volume of distilled water, mechanical aeration, and measurement of the oxygenuptake rate with a YSI 52 dissolved-oxygen meter equipped with a YSI 5905 BOD Probe electrode (APHA, 2710 B) [6]. Heavy-metal concentrations were determined according to the EPA SW 846 [7] standard procedures by the following methods: Statistics. Averages and standard deviations of calculated SVI were used to estimate the concentrations of the waste-sludge solids, as the SV30 divided by the average SVI. The results obtained from VS reduction, SOUR, and E. coli concentration in each cell were used to calculate the coefficient of variation along with the standard deviation, expressed as a percentage of the average value, in order to compare the performance and variability between cells. Statistical analyzes were performed with the free InfoStat software, version 2020 [10].

Results and Discussion
Waste Sludge. The table 1 summarizes the statistics obtained from the sampling of waste, applied and returned sludge, throughout the entire experimental period.

Source: Authors' own creation
Sludge loading. Through the use of the calculations indicated above on the basis of the ML concentration and an estimated 15 % return of TS with the clarified supernatant, the corresponding cell was loaded daily with 17-26 kg TS at an average of 21.6 kg TS per application. Accordingly, after 4 years of operation and 1465 applications -an average of 122 applications per cell-the system was loaded with a total of 2600 kg TS cell -1 , giving a total loading of 32,000 kgTS in the whole system, or roughly 130 kg TS m -2 , during those four years. These figures likewise represent an input of 32.5 kg TS m -2 y -1 . As mentioned above, during the first year of operation (commissioning), owing to different causes of solid losses, the estimated loading rate was 25 kg TS m -2 y -1 . That figure increased with time to reach a rate of 65 kg TS m -2 y -1 in the period of highest concentration. Nevertheless, the average loading rate was far below the maximum of 50-60 kg TS m -2 y -1 widely recommended in the literature [11].  [12] and 20 % by Nielsen [11]). The cells in the INP system were shallower (0.70 m) than the standard recommended by the technology -1.8 m in Nielsen [13]-and could not be charged for periods of 4 to 5 successive days because of the volume limitations. The cells were utilized by loading a different one each day, thus attaining a resting time of 11 days between loadings. Nielsen [11] reported a typical final TS concentration of 20 % in several Danish systems operated with 40 to 50 days of resting time between loadings, figures that could increase to up to 40 % with resting times of 60 to 75 days. In all likelihood, our results would be improved if longer resting periods could be attained with bigger cells. Another consideration with respect to the dehydration rate obtained is that the INP system works under a rainfall regime of almost 2 m y -1 , amount of precipitation that represents a 25 % extra volume of water to be evapotranspired in addition to the one added with the sludge. Nonetheless, the 14.3 % TS concentration obtained indicated the ability of the system to remove enough water from the sludge to produce a sufficiently solid material for easy handling with a small mechanical shovel. An additional advantage in this approach was that the residues analyzed contained final concentrations of 21.9 g total nitrogen and 8.4 g of total phosphorus per kg of TS (data not shown), enrichment that underscored the value of this system as a source of nutrients for soil amendment.

Vegetation. Although
Vector-attraction parameters. Annex II, in     The E. coli counts in the concentrated sludge applied to the cells ranged between 5 x 10 6 and 2 x 10 7 MPN L -1 (at an average of 1 x 10 7 MPN L -1 ). As indicated above, the average TS content of the ML was 0.14 % or 1.4 g TS L -1 . Thus we calculated the E. coli concentration in the input sludge to the cells to be 1 x 10 7 MPN L -1 divided by 1.4 g TS L -1 or 7.1 x 10 6 MPN g TS -1 . The average E. coli concentration in the analyzed residues was 1.0 x 10 2 MPN g TS -1 , figure that represents a reduction in bacterial (E. coli) concentration above 4 logs. Nielsen [13] reported a final concentration of less than 2 x 10 2 in the whole column of sediment beds in Helsinge, Denmark, after 3 months without loadings, while Uggetti [12] reported < 3 MPN g TS -11 of E coli in the sediments of Seva, after 4 months of resting. Our final concentration -4.7 x 10 1 to 2.3 x 10 2 MPN g TS -11represents between 5 and 23 % of the maximum value accepted by the National Resolution 410/18 of <10 3 MPN g TS -1 . These values and the absence of pathogens enable a classification of the processed sludges as Class-A biosolids, without use restrictions. According to title IV, art. 10, involving the forms of use and final disposal, these residues could be used for forestation and floriculture, the recovery of degraded sites, the restoration of landscapes and general landscaping, the development of fertilizers or amendments, the closure of sanitary fillers, and/or the development of construction materials.
The analysis performed on every individual cell residue revealed quite homogeneous results with precision well below 20 %. Within the vector-attraction parameters (table 2), the VS reduction indicated a coefficient of variation (CV) of 6.14 % and the SOUR a CV of 16.4 %; while the E. coli counts, after log10 transformation for normal distribution analysis, evidenced a CV of 13 % (table 4).

Conclusions
The experimental project carried out at the Iguazú National Park, Misiones, Argentina, demonstrated that the sludge treatment reed beds technology was successful in: (i) Reaching a dewatering (concentration) of waste sludges from 0.4 to 14 % TS (ii) Generating a well stabilized (low vector-attraction-potential) sludge, (iii) Generating a sanitary, safe sludge -at < 10 3 E. coli NMP g TS -1 , (iv) Generating a sludge enriched in N and P -at 21.9 g total nitrogen and 8.4 g total phosphorus per kg of TS, (v) Generating a Class-A-biosolids residue that, according to National Resolution 410/18 and to the Code of Federal Regulations, is reusable for agronomic and landscape purposes.
These results were obtained in a tropical climate of average annual temperature of 17 to 27 °C and a rainfall of 1870 mm y -1 and with the locally available vegetation cover Hymenachne donacifolia (Raddi) Chase and H. pernambucense (Spreng.) Zuloaga, quite different from most of the systems referenced in the literature, with those being carried out in temperate or cold climates. The present results suggest that the sludge treatment-reed beds technology is extremely versatile and with facile application to a wide range of climatic and biogeographic conditions. Moreover, the sludge treatment in the reed beds system produced other sanitary and economic benefits. Dry solids do not have to be shoveled and bagged, thus saving man hours and a sanitary risk for workers, and moreover they do not need to be transported to a safety landfill or disposed of in any manner. Finally, with this technology, the residue volume was significantly reduced by dehydration; and, at almost no energy cost, the wasted sludge stabilized and became 221 t (in 144 m 3 ) of a harmless product whose reduction in weight and bulk enabled the safe disposal and reuse in soil amendment.