Ano 7 (2020) – Número 2 Artigos




Amanda Nathália da Silvaa, Adilson Dalmoraa, Beatriz Firpoa, Rubens Muller Kautzmannb, Claudete Gindri Ramosc*


a Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves, 9500

Prédio 75, Sala 122, Campus do Vale, Porto Alegre, RS, CEP: 91501-970, Brasil.

b Universidade La Salle, Mestrado em Avaliação de Impactos Ambientais. Victor

Barreto, 2288, Centro, Canoas, RS, CEP: 92010-000, Brasil.

c Universidad de la Costa, Department of Civil and Environmental Engineering. Calle

15 58 #55 – 66, Barranquilla, Colombia.


*Corresponding author.

E-mail address: claudeterms@gmail.com



The present study focuses on the agronomic efficacy evaluation of volcanic rock mining waste and the sewage sludge from a dairy industry as a fertilizer for an acid soil, and to evaluate its nutrients and toxic potentially elements. The waste samples were collected from a volcanic rock mining and a dairy industry located in the southern region of Brazil. X-ray diffraction technique was employed to identify the mineralogical phases present in the rock dust. Rock and sludge major elements composition were analyzed after fusion along with LiBO2 followed by X-Ray Fluorescence. Toxic potentially elements content was measured according to United States Environmental Protection Agency 3050b method. The sludge and rock were applied to small-scale field experiment in which black oats was then sown. Four treatments were compared: (T1) 3,240 kg ha-1 of sewage sludge, (T2) 1,620 kg ha-1 of sewage sludge with 6,000 kg ha-1 of rock, (T3) 12,000 kg ha-1 of rock and control, which did not receive any type of input. Changes in soil properties and the nutritional status of the black oats were monitored after ninety days. The addition of the sludge combined with rock dust led to substantial increases in black oats leaves nutrient concentrations (mainly Ca, K, P and Zn) and in soil available K and P. In addition, the potentially toxic elements levels of both wastes are low and therefore the risks of environmental contamination are considerably reduced.

Keywords: dairy industry sewage sludge; volcanic rock mining waste; natural soil fertilization.



The increasing industrialization, since the Industrial Revolution and urbanization in an accelerated way allied with the capacity of humanity to intervene in nature, considerably increases the amount of waste generated in various industrial sectors. In the same way that it interferes generating damages to the environment, it offers sustainable techniques and procedures (Romeiro, 2003). Concern over the use of natural resources imposes rigorous legislation and more efficient monitoring (Varadarajan, 2015, Eggleston and Lima, 2015, Bhinge et al., 2015). About 20 years ago, the waste problem and its safe disposal did not receive much attention. The inadequate way they were disposed of led to water pollution and soil contamination, directly affecting human health and the environment (Lora, 2002). The need to meet quality standards for waste disposal becomes costly for industries and there are indications of industries moving from locality to areas with no rigid control standard in order to reduce their treatment costs (Braile and Cavalcanti, 1993). However, waste treatment and disposal are just some of the steps to be taken to develop environmentally sustainable production processes. Thus, several studies have evaluated the use of residues for nobler purposes, mainly for use in agriculture (Malafaia et al., 2016, Silva et al., 2016, Kumar et al., 2016 and Carey et al., 2016). Some of these residues are rich in nutrients and can be used in agricultural practice and are dependent on the process from which they have been found. This is an excellent alternative to reuse, which applies to sewage sludge from the dairy industry and the waste of rock mining volcanic rocks (Raij et al., 1996). These two materials are intended for disposal but can be used as natural soil fertilizers.

In Brazil, the dairy industry presented growth of 4.0% per year between 2000 and 2008, an above-average global average of 2.1% per year (Carvalho, 2010). The dairy industry uses water in abundance in all stages of manufacturing its products. The waters from the manufacturing processes of dairy products are composed by high nutrient concentrations such as nitrogen and phosphorus. If discarded in water bodies it can eutropize the water due to the excess of organic matter and other nutrients, as it will increase the concentration of aerobic bacteria, which will consume the dissolved oxygen of the water. This consumption can lead to the death of aquatic life, in addition to generating unpleasant odors due to the formation of hydrogen sulphide (Danalewich et al., 1998).

The nutrient replacement to the soil through the application of rock dust, known as stone meal or soil remineralization (Theodoro et al., 2010, Leonardos et al., 1976) is a process that corrects and positively alters the fertility index. This practice assumes that the rocks supply a nutrient demand in tropical soils, where there are periods of intense rains the industrial nutrients of origin would leach in a short time because they are extremely soluble. However, the nutrients provided by the rocks do not have high solubility, thus ensuring the availability of nutrients for longer. This may mean that the stone meal will not contaminate the environment by the application of rock dust as usually happens with industrial fertilizers, since what is not absorbed by the plants will be leached affecting the water bodies (Theodoro and Almeida, 2013).

Ramos et al., (2015) suggested carried out a mix with volcanic rock powder and other materials such as sludge from the dairy, aiming at modifications in the structure of the rock powder and largest release of nutrients in smaller time interval. According to Anjanadevi et al. (2016), nutrient solubilization of the rocks can be accelerated by mixing them with organic waste. That way, several studies have been carried out to simulate processes or techniques capable of optimizing the solubilization of inorganic residues (Sarma et al., 2016; Voisin et al., 2017) and/or to find applications of organic residues (Case et al., 2017).

However, due to the large amount of waste produced in southern Brazil annually, the main objective of the present work was to evaluate the potential use of volcanic rock dust (MRock) and dairy sludge (DSludge), combined or not, as natural soil fertilizer.

In this context, it is possible to affirm that the present study stands out due to its contribution to the reuse of two residues, such as rock dust and dairy sludge in the fertilization of acid soils.



The dacite sample (MRock) is of acid volcanic origin and belongs to the category of igneous rocks, formed by the cooling and solidification of magma, mostly composed of aluminum, iron, calcium, sodium, magnesium, silicon and potassium oxides (Gill, 2010).

An amount of 50 kg of material passing in a <0.6-mm sieve was supplied by the crushing plant of the Nova Prata city in Rio Grande do Sul (RS) state, Brazil.

The sewage sludge from dairy industry (DSludge) was the result of washing equipment from various parts of production such as: pipes, tanks, floor and other equipment involved in the production processes. A quantity of 600 L of DSludge was fed, after maturation for 7 days, by the dairy industry of the city of Nova Petrópolis, RS. DSludge did not receive any type of treatment in the laboratory before its disposal in the soil to be fertilized.

The experimental procedures and analytical conditions of mineralogic characterization of the MRock sample and MRock and DSludge major elements composition were described by Ramos et al. (2015). DSludge and MRock toxic potentially elements (PTEs) content was measured according to United States Environmental Protection Agency 3050b method (USEPA, 1996).

Typic Hapludox (LSoil) was analyzed, before and after planting, for phosphorus and potash (Mehlich 1), calcium, aluminum and magnesium (KCl 0.1 M L-1), zinc and cooper (HCl 0.1 mol L-1) and pH in water (1:1) according to Tedesco et al. (1995).

Four soil plots, 10 m² each in a 1 by 10 m design, were prepared in Santa Rita county, RS, Brazil. Each plot was divided into 4 parts of 2.5 x 1.0 m, totaling 16 sample units (Figure 1).

Figure 1. Experimental overview.


The proportions used in the treatments of plots 1 to 3 were based on the fertility of LSoil and on the content of K2O present in DSludge (0.1%) and MRock (3.31%). The description of the treatments is presented in Table 1.


Table 1 – Treatments applied to LSoil.


Treatment LSoil DSludge MRock
Control 10 m²
T1 10 m² 3,240 kg ha-1
T2 10 m² 1,620 kg ha-1 6,000 kg ha-1
T3 10 m² 12,000 kg ha-1


After amendment application (treatments), soils were sown with black oats seeds which was cut 90 days after.

Plant tissue, right after cut, and leafs, steam and roots were separated and washed with lab water. Plant tissue was dry oven at 60º C until constant weight, were ground in a knife mill and digested according to the methodology proposed by (Jones et al., 1991) and sent for analysis of Ca, Mg, K, P, Cu and Zn contents in the leaves.

All data were submitted to analysis of variance, using the Tukey test at 5% probability. The software used was SPSS statistics version 20.



DSludge and MRock characterization


The mineralogical composition obtained by X-ray diffraction revealed that the sample contains 15% quartz, 54% anorthite, 19% sanidine, 1% cristobalite and 10% augite (Figure 2).

Figure 2. Diffractogram of X-rays of the MRock.


Table 2 shows the major elements concentration of the DSludge and MRock determined by X-ray fluorescence (XRF) and equivalent value in g kg-1.


Table 2 – Major elements concentration of DSludge and MRock.


Oxides DSludge MRock
% g kg-1 % g kg-1
Al2O3 3.90 20.6 14.3 75.7
CaO 0.70 5.00 3.56 25.4
Fe2O3 0.50 3.50 6.56 45.9
K2O 0.10 0.80 3.31 27.5
MgO 0.10 0.60 1.27 7.70
MnO 0.13 1.00
Na2O 0.40 3.00 3.13 23.2
P2O5 1.90 8.30 0.26 1.10
SiO2 64.8 303
TiO2 0.86 5.20
LOI 1.35


The data in Table 2 show a high K content in the MRock possibly due to the presence of sanidine, which is a mineral that makes up 19% of the composition of this rock. The content of Na must be related to the presence of anorthite, sanidine and augite (minerals containing Na in its composition). The Mg content must be related to the presence of the augite that constitutes 10% of the MRock. The anorthite that is a calcium plagioclase and represents 54% of the mineralogical composition of the MRock should be the mineral responsible for the high Ca content in the MRock. As expected, Fe and Al contents were high because these elements make up most of the minerals that form volcanic rocks (Gill, 2010). In addition, the data in Table 2 agree with the mineralogical composition of the MRock, detected by XRD analysis.

Table 3 shows PTEs concentrations of DSludge and MRock.


Table 3. Potentially toxic elements concentration of DSludge and MRock.

Elements Sludge Dacite Brazil (2006)

SS limit

USEPA (1999)

SS limit



SS limit

mg kg-1
As <0.60 3.0 41.0 75.0
Cd <0.10 0.08 39.0 85.0 40.0
Cr <0.02 7.0 1,000 3,000 1,000
Hg <0.10 <0.01 17.0 57.0 25.0
Pb 7.78 18.7 300 840 1200
Se 0.53 0.08 100 100


According to Table 3 it was possible to note that the PTEs levels of both residues are low and therefore the risks of soil contamination and entering the food chain are considerably reduced. The dairy sludge potentially toxic elements levels are below the levels permitted for sewage sludge by all studied legislations (Table 3). The results concur with the studies of López-Mosquera et al. (2002), who reported a similar concentration in dairy sludge of Spain. In Brazil, there is a legislation with well-defined specifications for the use of rock powder in soil fertilization. These include the maximum permitted limits for PTEs, which are 15, 10, 0.1 and 200 ppm for As, Cd, Hg and Pb, respectively (Brazil, 2016). Table 3 shows that the PTEs levels in dacite are below the limits allowed for its application in Brazilian agricultural soil. These results agree with Suárez et al. (2004) and Van Straaten (2007) which the application of these wastes in agricultural soils is a safe alternative for their discards.

Agronomic performance of DSludge and MRock

Table 4 shows the results of the soil fertility analysis before the application of the treatments and after the harvest of the black oats plants.


Table 4. Means results of fertility analysis of LSoil and soils collected after black oats harvest*.

Treatments pHH2O Al Ca Mg K P Cu Zn
cmolc dm³ mg dm-3
LSoil 4.7a 0.9a 1.5b 0.5a 44b 7.9c 3.7a 4.4a
Control 4.2bc 0.4b 1.4b 0.6a 50ab 20b 3.9a 4.4a
T1 4.3bc 0.4b 2.3a 0.8a 53b 18b 3.9a 4.3a
T2 4.1c 0.4b 2.2a 0.7a 65a 30a 3.8a 4.4a
T3 4.5ab 0.2c 1.8ab 0.7a 57ab 20b 3.7a 4.2a

*Values followed by a different letter within a column are significantly different at P < 0.05.


Typically, prior to planting, some inputs such as limestone, single or triple superphosphate and/or KCl are added to soil with low fertility to correct the acidity (pH), phosphorus and potassium levels, respectively. It is emphasized that the soils of this experiment did not receive liming or any type of inputs to correct their characteristics before the application of the wastes.

The pH values of all soils, which were already considered low, presented a small tendency to decrease after the cultivation of black oats (Table 5). This indicates that none of the treatments were enough to reach pH 6.0, which is considered adequate for the cultivation of most agricultural crops according to the Brazilian Soil Science Society (SBCS, 2004). Possibly, this is due to the low carbonates content in MRock (Queiroz, 1980) and the low release of Ca and Mg by both wastes.

Table 5 shows that the application of MRock significantly reduced Al levels in soil of T3 treatment, after black oats crop. Moreover, despite the presence of Al2O3 high levels in MRock it was evidenced that Al was not available for the soil even under acidic conditions (pH ≤ 4.7). According to Faquin (2005) and Linsay (1979), in pH > 5.8 there is no Al reaction in the soil. This result is relevant, as it supports the sustainable agricultural use of the MRock after it is applied in tropical soils that are normally weathered and acids due high Al levels.

The available Ca contents did vary significantly across the different treatments, especially after sludge application in T1 and T2 treatments (Table 5). These treatments were able to increase the calcium level of the soil and to change the interpretation of this nutrient that was considered low (Ca  ≤ 2.0 cmolc dm-3) and became considered medium (SBCS, 2004).

The available Mg did not vary significantly over the different treatments, although there was a trend towards higher Mg levels after application of the sludge (Table 5).

The P contents were greatly affected by T2 treatment with DSludge and MRock doses. Thus, there is a clear tendency to increase the total phosphorus content due to the combination of DSludge and MRock (Table 5). Phosphorus plays an important role in the growth of the root system, as well as in the tillering of grasses, which are fundamental to the higher productivity of forages. According to Holford (1997), phosphorus is the second essential nutrient most limiting to agricultural production, after nitrogen, in tropical soils. Therefore, the use of DSludge combined with MRock, as natural fertilizer especially in phosphorus, becomes particularly relevant because, according to Santos et al. (2002), tropical soils present low natural availability and high adsorption and nutrient fixation capacity.

All treatments resulted in potassium levels higher than those found prior to the implementation of the experiment, especially in T2 treatment (Table 5). DSludge typically has low K content (Hue and Ranjith, 1994), so that K supplementation may be necessary for only DSludge fertilized soils. Therefore, DSludge and MRock dose (treatment T2) can provide an alternative to the use of conventional fertilizers, for tropical soils. In addition, this treatment improved the availability of nutrients, especially P and K, without producing adverse effects on soil fertility (Figure 3).

Figure 3. Black oats crop after 90 d of planting.


The Cu and Zn contents were not greatly affected by any treatment and the values remained high (> 0.4 and > 0.5 mg dm³), respectively (SBCS, 2004).

The nutrients accumulation in the leaves were evaluated to verify the efficacy of DSludge and MRock as a natural fertilizer to optimize its uses. The results are shown in Table 5.


Table 5. Averages of nutrients in black oats leaves and adequate range (Pauletti, 2004).

Treatments Nutrients*
Ca Mg K P Cu Zn
(g kg-1)  (mg kg-1)
Control 4.52d 2.60c 28.6c 4.97d 7.96d 55.6d
T1 5.98b 2.75b 29.5b 5.14c 11.9c 62.7c
T2 6.85a 2.87a 31.4a 7.49a 16.9a 69.6a
T3 5.90c 2.82a 29.5b 6.67b 13.2b 68.7b
Adequate range 2.50-5.00 1.50-5.00 15.0-30.0 2.00-5.00 5.00-25.0 15.0-70.0

*Values followed by a different letter within a column are significantly different at p < 0.05.


The plants cultivated in the T1, T2 and T3 treatments had significantly higher levels of phosphorus than the control treatments (p < 0.05). The higher soil P content occasioned by DSludge and MRock application led to significative increase (p < 0.05) in P level in the leaf of the plants, with value reaching 7.49 g kg-1, above the adequate range for black oats (Table 5). This shows that the combination of the two wastes studied is a potential source of phosphorus for agriculture.

Leaf Ca, Mg and K concentrations were higher in the (T2) DSludge and MRock treatment. The Ca and Mg concentrations recorded in the leaf of the plant were like the reference values indicated by Pauletti (2004). In addition, the levels of Ca, P and K in the leaves of black oats were higher than those of Suárez et al. (2004). These authors applied doses of 80 and 160 m3 ha-1 of dairy sludge in humic cambisol and cultivated Lolium multiflorum for nine weeks. The results of Ca, P and K absorption in the aerial part of the plants at the lowest dose were 4.54, 2.51 and 8.91 mg g-1, respectively.

Although all soils showed high levels of Cu and Zn, black oats of all treatments absorbed quantities of these micronutrients within the adequate range (Table 5).

Therefore, the results obtained in this study made clear that the addition of DSludge favored the dissolution of the minerals contained in MRock, corroborating the assumption of Ramos et al. (2015), evidencing that these combined wastes were effective in the natural fertilization of the soils and in the supply of nutrients to the plants. This is in fact one of the most important points in the achievement of such goals, i.e., if the primary minerals are really being destroyed/weathered and further releasing trace elements to the soil-plant system.



The results of the present study confirmed that the application of DSludge with the combined MRock improved soil fertility and had an immediate effect on the nutrient supply to the black oats crop favoring the nutritional state of the plants.

Therefore, the application of MRock and DSludge mix proved to be as a sustainable technology for natural fertilization of soils.

The PTEs contents of both wastes are low and therefore the risks of environmental contamination are considerably reduced.

This study showed that the application of the combined wastes in the soil proved to be a safe alternative for its disposal.

For future studies, it is intended to evaluate the application of rock mining waste and sewage sludge from dairy industry in greenhouse with black oats and maize crops to verify the residual effect of the rock after cultivation.



Authors acknowledge to Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS), Edital 014/2012 – BMT for funding.



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