Collection and analysis of industrial sludge

The sludge used was from a denim manufacturer: Fabrica la Estrella S. A. de C. V. from Parras de la Fuente, Coahuila, Mexico. These were obtained from the filtrate and solid pressing hauled in wastewater from the industrial process. Sludge or solids count with certificate of non-hazardous based on the analysis of CRETIB (corrosivity, reactivity, explosidad, toxicity, flammability and biological-infectious) of the General Law of Ecological Balance and Environmental Protection from SEMARNAT (^{SEMARNAT, 2014}).

To verify and document the presence and concentration of fecal coliforms, Salmonella spp., helminth eggs and elements: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc, the sludge was analyzed in the laboratory Intertek Testing Services de Mexico , S. A. de C. V., accredited by the Mexican Official Standard NOM-004SEMARNAT-2002 (^{SEMARNAT, 2003}). Additionally, the physical characteristics salinity, sodicity and fertility were determined according to Mexican Official Standard NOM021-RECNAT-2000. (^{SEMARNAT, 2002}).

Plant material

Ornamental plant rooted cuttings from hydrangea (Hydrangea macrophylla L.) var. Blue Fantasy were used.

Research site

The study was conducted in a semicircular greenhouse with temperature control from the Horticulture Department of the Universidad Autonoma Antonio Narro, Buenavista, Saltillo, Coahuila, Mexico located at a latitude of 25^o north and 23 'and west longitude 101° 02', with an altitude of 1 743 masl.

Design and experimental treatments

Using peat (Peatmoss^®) as base, four treatments with different sludge concentrations 0%, 10%, 20%, 30% (v/v) were established. Once the mixtures were ready, they placed in containers or pots of 2 L, with rooted cuttings of hydrangea. The pots were placed randomly in the greenhouse. 15 replications per treatment and each replication was a pot. For irrigation and nutrition, solution Steiner (^{Steiner, 1961}) was used at 50% of the plantation until the appearance of flower bud and 75% after flower bud to harvest flower.

Variables evaluated

Electrical conductivity and pH in leachate. To verify the effect of industrial sludge inclusion on leachate in each treatment, five samples at 15, 30, 45, 60 and 75 days after transplantation were made, for this purpose a plastic container was placed under the pots, three per each treatment, the sample was collected in the morning, in which pH and electrical conductivity was measured with a potentiometer HANNA model Combo HI98120.

Plant height and inflorescence diameter. Evaluations were made at 118 days after establishing the experiment, when the plants were at flowering. Plant height was measured from the base of the plant to the floral apex and the diameter of the inflorescence was measured using a longitudinal-transverse projection, i.e., two measurements were made in a cross north-south and east-west axis per inflorescence and the average was obtained.

Nutrient concentration in the plant. Once harvested plant leaves, stems, flowers and roots were removed and placed in a forced air oven 72 hours at 70 °C; then an analysis of K, Mg, Ca, Fe, Mn, Cu, Zn and Na concentration in each of the organs sampled using an spectrophotometer of atomic absorption Varian AA-1275 (^{Fick et al., 1976}). Total nitrogen was measured with micro Kjelhdal (^{AOAC, 1980a}), and phosphorus through spectrophotometry (^{AOAC, 1980b}).

Chlorophyll concentration in leaf tissue. It was determined through spectrophotometry (^{Harborne, 1973}), at 660 and 642 nm in an absorption spectrophotometer Bausch & Lomb, Spectronic 21.

Analysis of industrial sludge

Microbiological analysis. The sludge for this study was classified as type A, for its low content of fecal coliforms, absence of Salmonella and helminth eggs (Table 1). Being possible its use for agricultural purposes. It is also possible direct manipulation without risk to man and plants, according to the Official Mexican Standard NOM-004SEMARNAT-2002 (^{SEMARNAT, 2003}).

Table 1 Microbiological analysis of textile industrial sludge, factory La Estrella S. A. de C. V. of Parras de la Fuente, Coahuila, Mexico. 

NMP= Número más probable; ND= No detectado; HH= Huevos de helminto.

Analysis of heavy metals and metalloids. The results of the analysis (Table 2), classified the sludge as excellent according to NOM-004-SEMARNAT-2002 (^{SEMARNAT, 2003}), since its concentration is below the maximum allowable limit. The management of sludge involves no risk.

Table 2 Content of heavy metals in the textile industrial sludge, factory La Estrella S. A. de C. V. of Parras de la Fuente, Coahuila, Mexico. 

ND = No detectado.

Physical analysis of salinity and sodicity of sludge. Analysis results indicate clay loam, bulk density of 0.74 g cm^-3 and organic matter content 2.16%, carbonate 8.98% and moisture retention at field capacity of 30.45%. In relation to salinity, sludge has an extremely high electrical conductivity and alkaline pH according to NOM-021-RECNAT-2000 (^{SEMARNAT, 2002}). High sulfate, bicarbonate and chloride values, with no presence of carbonates, medium in potassium, high calcium, magnesium and sodium (^{Richards, 1980}) (Table 3). Due to high salt content in the sludge it was necessary to use them in mixture with peat to dilute the salts present in the sludge. Previous studies indicated that the maximum content of textile sludge based on volume is 25% (^{Benavides-Mendoza et al., 2007}).

Table 3 Analysis of salinity and sodium saturation extract textile industrial sludge, factory La Estrella S. A. de C. V. of Parras de la Fuente, Coahuila, Mexico. 

Fertility analysis. The concentrations of mineral elements in textile sludge were very low and moderately low for N, P and S. As for Ca, Zn, Fe and Mg had very high values (^{SEMARNAT, 2002}) (Table 4). Mineral content in sludge does not indicate the possibility of restriction to plant growth, by excessive concentration of elements. Moreover, by mixing the sludge with the substrate in concentrations below 25% (^{Benavides-Mendoza et al., 2007}), the mineral content decreases, being in low or very low ranges.

Table 4 Analysis of fertility textile industrial sludge, factory La Estrella S. A. de C. V. of Parras de la Fuente, Coahuila, Mexico. 

ND = No determinado.

pH and electrical conductivity (EC) of water leached

Sludge inclusion in the substrate, increased pH and EC of leached water or nutrient solution (Table 5). Applying a correlation analysis, it was observed that there is a direct relationship between the percentage of sludge inclusion to the substrate and the increase of pH (r= 0.74) and EC (r= 0.76) in the leachate or nutrient solution. Alkaline pH in the substrate reduces the availability of nutrients and the increase of EC is related to high salt content in the substrate, which comes from products used to apply and fix colors in textile (^{Benavides-Mendoza et al., 2007}).

Table 5 Mean and standard deviation of the pH and EC of the water obtained leached acidic peat substrate and textile industrial sludge at different concentrations. 

Medias con distintas letras son diferentes estadísticamente (Tukey P ≤ 0.05).

Morphological variables

Sludge inclusion on the substrate, affected plant height and inflorescence diameter (Table 6). The reduction in plant growth and inflorescence, could relate to the increase in EC generated by the addition of the sludge. This effect is tangible by verifying that the concentrations of 20% and 30% increased leachate EC to 4.52 and 4.36 dS m^-1 respectively, while the control was 2.16 dS m^-1. EC values greater than 4 mS cm^-1 reduces growth and production in sensitive plants (^{Maiti and Benavides, 2002}) and in particular ornamental, either woody (Aguilar Valdez et al., 2011), or herbaceous like buttercup (^{Valdez Aguilar et al., 2009b}.), lisianthus (^{López-Pérez et al., 2014};. ^{Valdez Aguilar et al., 2013}) and cempazuchitl (^{Valdez Aguilar et al., 2009a}); also semihardwood ornamentals like hydrangea, are more susceptible to salinity than herbaceous (^{Cabrera, 1999}).

Table 6 Mean and standard deviation of plant height and diameter of inflorescence hydrangea, grown in acidic peat substrate and textile industrial sludge at different concentrations. 

Medias con distintas letras son diferentes estadísticamente (Tukey p ≤ 0.05).

However, other studies found that the addition of sludge to soil or substrate had a positive effect on Lilium sp. (^{Torres-González et al., 2011}), lettuce (^{Vaca et al., 2004}) alfalfa (^{Uribe et al., 2001}) and maize (^{Salcedo et al., 2007}). The different responses of plants are related to physical and chemical properties of the sludge and these vary according to their origin: urban or industrial and type of process to which these are subjected (^{Cooper, 2005}).

Absorption of mineral nutrients

Macronutrients. N, Mg and Ca, showed no difference between treatments for the different organs of the plant, the same thing happened for K in leaves, while the latter increased its concentration in the inflorescence and stem. As to P, this showed negative trend in the stems and roots regarding the concentration of the sludge, while inflorescence and leaves showed a slight increase in 10% of sludge (Table 7). ^{Mills and Benton (1996)} reported that the percentage content recommended for macronutrient in leaves of H. macrophylla is: N 2.24 - 5.6, P 0.25 - 0.7, K 2.2 - 7.8, Ca 0.6 - 2 Mg 0.22 to 0.6.

Table 7 Concentration of macronutrients in different organs hydrangea grown in acidic peat and textile industrial sludge at different concentrations. 

Medias con distintas letras son diferentes estadísticamente (Tukey p ≤ 0.05). CV = Coeficiente de variación. Para fósforo no hubo repeticiones por lo cual no se realizó análisis estadístico.

According to nutrient content recommended by ^{Mills and Benton (1996)} and the concentrations observed in this study, plants were in the range of sufficiency except for potassium showing values below the recommended level. Even if potassium was generally low, the highest concentration was found in inflorescences, leaves and stems, but in root this effect was not observed. In stem, potassium content increased in proportion to the concentration of sludge in the substrate, which seems to indicate that the sludge promotes the absorption of potassium, even when, in previous studies low levels of potassium are reported in this type sludge (^{Narváez et al., 2013}). Also, the reports from the literature show that sludge affects in different ways nutrient uptake by plants, for example, it is reported that the addition of sludge decreased Ca concentration in tobacco leaves (^{Kabata and Pendias, 1992}) and in tomato fruits (^{Narvaez-Ortiz et al., 2013}).

Micronutrients. The response in the concentration of trace elements varied according to the organ analyzed. In root and stem, Fe increased in sludge concentration 20%, the other nutrients were similar to control. On leaves and inflorescences Na and Zn showed no difference regarding the control, while Cu increased in all industrial sludge treatment. ^{Mills and Benton (1996)} report that proper micronutrient content in mg kg^-1 for H. macrophyla is: Fe 50-300, Mn 38-300, Cu 1-25, Zn 20-200 and according to the results of this study micronutrient content is the sufficiency range. As it has been mentioned that sludge is a source of mineral nutrients (^{Castro et al., 2007}; ^{Barbarick et al., 2012}), noting the concentration of micronutrients in textile sludge, it seems that there is no relation between them and the obtained in the different plant tissues. Therefore it is possible that the requirement of micronutrients has been provided by the Steiner nutrient solution (^{Steiner, 1961}), not allowing the expression of nutrients from the substrate (Table 8).

Table 8 Concentration of micronutrients in different organs hydrangea grown in acidic peat and textile industrial sludge at different concentrations. 

Medias con distintas letras son diferentes estadísticamente (Tukey p ≤ 0.05). CV = Coeficiente de variación. Para fósforo no hubo repeticiones por lo cual no se realizó análisis estadístico.

Chlorophyll content

The inclusion of textile industrial sludge in the substrate did not cause any changes in chlorophyll concentration (Table 9). Chlorophyll content is an indicator of photosynthetic activity in plants (^{De Jong et al., 1984}), and is related to nutrients content and environmental conditions of light and temperature (^{Salisbury and Ross, 2000}). The experiment was conducted in greenhouse where temperature conditions, radiation and humidity were the same for all treatments and it was verified that nutrient concentration in the plant were in the sufficiency range. This suggests that chlorophyll content in plants was more related to nutrient concentration and environmental conditions than to EC and pH of the substrate. Additionally, it was found that total chlorophyll content in leaves of hydrangea, was similar to that reported in orange leaves (^{Reyes-Santamarina et al., 2000}) and lower to that assessed in tomato leaves (^{Rodríguez et al., 1998}).

Table 9 Mean and standard deviation of the chlorophyll content in leaves grown in acidic peat hydrangea and textile industrial sludge at different concentrations. 

Medias con distintas letras son diferentes estadísticamente (Tukey P≤ 0.05).