Introduction

The sea contains large amounts of resources, including the seaweed that is washed up by tides and winds onto shores. Such drift seaweed has been used for centuries as a natural fertilizer in many coastal areas throughout the world (Zemke-White and Ohno 1999). Seaweed is particularly rich in potassium, micronutrients, growth activators, and also alginates, which can improve the soil structure (Blunden 1991). Seaweed is used for numerous purposes, and as it also forms the base of the food chain, it must be used sustainably. However, in some cases, seaweed is considered as waste, particularly in areas where green tides appear as a result of eutrophication. Green tides damage the touristic appeal of beaches and cause problems in shellfish-growing areas and in aquaculture (Morand and Briand 1996).

Fish waste has also traditionally been used as a fertilizer in coastal areas, as it is rich in nutrients, particularly nitrogen and phosphorous (Arvanitoyannis and Kassaveti 2008). Different types of fertilizers are made from fish meal, which has been authorized for use in agriculture, including ecological agriculture (EC Council 834/2007 2007). The European Union (EU) is the world’s fourth largest fish producer, and within Europe, Spain is the leading fish producer (capture fisheries and aquaculture produce 1,020,908 t  year−1 fresh weight, representing 15.84 % of the total production in the EU-27) (Unión Europea 2012) and also the leader as regards transformation of the raw material. Most of the fish captured in Spain are landed by Galician fishing fleet (NW Spain), which represents 40 % of the Spanish fishing fleet and mainly catches horse mackerel. The fishing industry generates large amounts of by-products, which depending on the type of transformation, may account for up to 30–45 % of the initial weight of the product.

With the aim of ensuring a more responsible and sustainable use of fishery resources, the size of the EU fishing fleet and the volume of fish captured have been reduced at an almost constant rate in recent years. The next reform of the Common Fisheries Policy proposes a ban on discards, so that larger amounts of waste/by-catch will probably be brought ashore. Large amounts of waste are also generated by the canning industry, fresh fish manufacturers, fish markets, and ports, and therefore, new ways of utilizing all of the waste must be found. This type of waste is usually used to produce fish meal, which is the way of use that causes a lower value.

Both seaweed and discarded fish are used to make products with a high added value (pharmaceuticals, cosmetics, functional foods, etc.), although only small amounts of the waste material are used. An alternative way of utilizing large quantities of these by-products would be to use them as amendments or fertilizers to improve and fertilize agricultural land or to use them as plant substrates. In areas where large amounts of seaweed appear as the result of eutrophication episodes, the seaweed is composted to produce fertilizers, as one of the most appropriate waste management methods from both economic and environmental viewpoints. This process reduces the volume of seaweed that reaches shores and produces good quality compost that is rich in nutrients, particularly potassium, calcium, and magnesium, and is also hygienic and free from contaminants, such as heavy metals and phytotoxic compounds (Eyras and Sar 2003). In different parts of the world, composting trials have been carried out with fish waste (usually generated on fish farms) as an alternative, viable method of transforming the waste into products that can be used in agricultural systems (Liao et al. 1997).

In order to produce optimal conditions, co-composting of different materials with complementary characteristics is often carried out. In this case, mixing seaweed, which is rich in potassium and micronutrients, with fish waste, which is rich in nitrogen and phosphorus, should produce a fertilizer containing the main essential elements for plants. However, given the low ratio of C/N in both types of material, a material rich in lignocelluloses must be added to provide aeration and as a source of carbon. Pine bark is a lignocellulosic waste material and, moreover, it is readily available in Galicia and the rest of northern Spain. According to IFN3 1997–2007 (MAGRAMA), the area covered by forest in Spain is the second largest in the EU, with a total of 26.3 million ha of land dedicated to forestry, i.e., 52 % of the total land area. In Galicia, some 69 % of the land is dedicated to forestry, and the dominant species are Pinus pinaster, Quercus robur, and Eucalyptus globulus, which generate more than 30 million m3 of bark a year.

Drift seaweed, fish waste, and pine bark are readily available in coastal areas in Galicia. The compost produced from these materials would be a fertilizer of natural origin and, if it complied with established criteria, could be used in ecological agricultural systems.

At the end of 2009, some 9.3 million ha of land was dedicated to organic agriculture in the EU, which was the second producer worldwide (Willer and Kilcher 2011). Within the EU, Spain is the country with the largest area of land dedicated to organic agriculture (1,650,886 ha in 2010) (MAGRAMA 2011). Organic agriculture promotes nutrient recycling, thus conferring organic matter a major role in maintaining the fertility of the soil–plant systems and avoiding the use of man-made products. However, despite the development of organic farming, there is still a scarcity of good quality organic fertilizers that can be used in organic production. On the other hand, the demand for organic products, at both local and worldwide scales, is increasing and future perspectives for this type of agriculture are very promising.

The aims of the present study were to make compost from waste products of marine origin and to evaluate the compost for use as an amendment, fertilizer, or ecological substrate.

Material and methods

Compost materials

The algal material used in the composting trial was drift seaweed collected from a beach on the coast of Lugo (NW Spain, 43°36′ N, 7°18′ W). The predominant species were brown seaweeds of the genera Laminaria spp. and Cystoseira spp. The fish waste was obtained at the same site, from a fish-processing factory where different species of oily fish are filleted. The waste (fish heads, skin, and spines) was derived from the horse mackerel Trachurus trachurus L. Both seaweed and fish waste are considered as category 3 type waste, i.e., it is suitable for use in agriculture after being composted or silaged (EC Council 1069/2009 2009). Pine bark (1–15 mm) was also added to improve the structure and increase the C/N ratio of the final product. The material was obtained from a pine-processing factory in the same area and it was not chemically treated after felling according to EC Council 889/2008 (2008).

The main characteristics of the materials are shown in Table 1. The seaweed supplied most of the moisture content, intermediate amounts of N, low amounts of P, and particularly high amounts of K and Na. The value of the C/N ratio was close to 17, which is consistent with the values reported from benthic brown algae collected from Galician shores (Villares et al. 2007).

Table 1 General characterization of the raw materials
Full size table

The fish waste contained high amounts of N and P, was highly saline, and had a low C/N ratio. As the C/N ratios in seaweed and fish are lower than the values considered optimal for composting (25–35) (Jhorar et al. 1991), a lignocellulosic material (pine bark) with a high C/N ratio was added to increase the C content and also to improve the aeration.

Setting up the composting trial

The trial was carried out in the open air, in a field close to the site where the material was collected. Three conical piles (6 × 2 × 1.5 m) of mixtures of the different materials were established by layering the seaweed, fish waste, and pine bark at a volumetric ratio of 1:1:3, as previously established (López-Mosquera et al. 2011). The piles were constructed on an impermeable base and covered with geotextile fabric (TopTex®) to avoid moisture loss.

Monitoring the compost process and sampling

The compost piles were turned weekly during the first 6 weeks and fortnightly during the remaining 4 weeks of the process. The piles were turned with an excavator. Water was not added throughout the process. The temperature was measured (in triplicate) during the process, at various points around the piles, at a depth of 45 cm. Likewise, samples of the material were collected, immediately before the piles were turned, at various points half way up the pile to measure the moisture content, pH, EC, C, and N.

Characterization of the materials and the final compost

Three composite samples of each of the materials were obtained at the sites of origin and three composite samples were obtained from each of the compost piles at the end of the process, when the compost was stable. The following parameters were measured in the raw materials: moisture content, pH, EC, and total C, N, P, K, Ca, Mg, and Na. In addition to these general parameters, the degree of stability (DS), phytotoxicity, and hygiene of the compost were tested, and the following chemical and physical properties were measured: particle density (PD), bulk density (BD), total pore space (TPS), air capacity (AC), easily available water (EAW), water buffering capacity (WBC), unavailable water (UW), cation exchange capacity (CEC), micronutrients (Cu, Fe, Mn, and Zn), potentially toxic metals (Cr, Hg, Ni, and Pb), and N forms (organic, ureic, nitric, and ammoniacal).

Physical parameters of the compost

The BD, PD, and TPS were determined following standard methods (AENOR 2001). The data for the water retention curve was obtained by subjecting the different samples to increasing pressure in a sandbox (De Boodt et al. 1974). The measurements were made at pressures exerted by water columns of 10, 50, and 100 cm, and the contents of the different forms of water were expressed as the volumetric percentage (v/v). The AC (v/v) is the difference between the TPS (v/v) and the volumetric percentage of water at a pressure of 10 cm; the EAW is the volume of water released by the substrate when the pressure is increased from 10 to 50 cm; the WBC is the volume of water retained by the compost at a pressure between 50 and 100 cm; and the UW is the volume retained by the compost at 100 cm of water (Felipó et al. 1979).

Chemical parameters of the starting material and the compost

The moisture content of the compost and raw material was measured as the weight lost after drying at 105 °C to constant weight. Carbon and nitrogen were determined after combustion in a Leco 2000 autoanalyzer. After acid digestion with H2SO4 and 30 % H2O2 (Thomas et al. 1967), the concentrations of Ca, Fe, Mg, Mn, and Zn were determined by atomic absorption spectroscopy and those of Na and K by atomic emission spectroscopy. The concentrations of P were determined by colorimetry (Chapman and Pratt 1997). The samples were subjected to microwave-assisted digestion with nitric acid (Microwave Labstation ETHOS 900), and the total concentrations of cadmium, copper, chromium, lead, mercury, and nickel were determined by inductively coupled plasma–optical emission spectroscopy. The aqueous extracts (compost/water, 1:5 v/v) were also obtained by UNE-EN methods 13037 and 13038 (AENOR 2001). The pH, electrical conductivity, and concentrations of calcium, magnesium, sodium, and potassium were determined in these extracts. The elements were analyzed by absorption and atomic emission spectroscopy. Nitrate and ammonium concentrations were measured with selective electrodes. All parameters were determined in triplicate and the data shown are the mean values. Data from these analyses were compared with the data for reference materials CRM 279. The percentage recoveries were satisfactory.

The nitrogen was fractionated into ureic, nitric, ammoniacal, and organic N, following (RD 1110/1991 1991), and the values were expressed as percentages.

Phytotoxicity test

Phytotoxicity was determined by the test described by Zucconi et al. (1981), with lettuce Lactuca sativa seeds. The stability of the product was evaluated after 2.5 months. The stability was tested indirectly by the Dewar flask or self-heating method (Brinton et al. 1995). The DS was determined by the method of Soliva et al. (2004), which consists of two successive hydrolysis procedures that enable estimation of the resistant organic matter (ROM) in the sample. The result can be expressed as the percentage of total organic matter (TOM) comprised by ROM: DS% = (ROM × 100 / TOM).

Sanitization

As required by the Spanish law concerning fertilizers, the levels of Escherichia coli and Salmonella in the compost were determined by the recommended methods (RD 824/2005 2005).

Desalination of the fish waste

The salinity of the fish waste was lowered by a desalination test procedure. For this purpose, the Trachurus trachurus L. fish waste was homogenized (<15 mm), and ten replicate mixtures comprising 500 mL of the homogenized fish waste and the same amount of distilled water were prepared (1:1 v/v). The EC of the resulting extract was measured at intervals of 30 min.

Statistical analysis

Data were analyzed for treatment effects by one-way analysis of variance (ANOVA). Prior to the ANOVA, the normality and homogeneity of the variances were checked by the Kolmogorov–Smirnov and Levene tests, respectively. Significant differences between the treatment means were calculated by the Tukey test, at p < 0.05. All analyses were performed with SPSS software 2009.

Results and discussion

Monitoring the composting process: temperature, moisture content, C/N, pH, and EC

The process followed a similar pattern in all three piles; the thermophilic stage was reached rapidly, and the maximum temperature was reached after 10 weeks (Fig. 1a). The thermophilic stage lasted for approximately 45 days. Temperatures above 55 °C for more than 30 days reduce and may even eliminate pathogens, thus ensuring the safety of the material (US Environmental Protection Agency (USEPA) 1993). After the maximum temperature was reached, the piles then cooled slowly until they were close to ambient temperature.

Fig. 1
figure 1

Changes in different parameters in the three piles during the composting process in comparison with optimal ranges: a temperature, b moisture content, c C/N ratio, d pH, and e electrical conductivity

Full size image

The moisture content of the three piles was similar throughout the process (Fig. 1c), and although no water was added during the trial, it was maintained at between 40 and 60 %, as recommended by Day et al. (2001). The presence of mucilaginous substances in the seaweed and the geotextile cover placed over the piles helped retain the moisture at optimal levels. The fabric cover reduced water loss by evaporation, while allowing gas exchange.

The initial C/N ratio in all three piles was between 23 and 29, which is adequate for starting the composting process (Fig. 1b). The mean value at the end of the composting process was 21.9 (Table 4).

The changes in pH followed similar patterns in all three piles until finally stabilizing at around pH 7.0 (Fig. 1d). The variations in pH were due to acidifying processes (release of CO2 and small organic acids), which occurred during the first days, and alkalizing processes (release of NH3 and loss of organic acids), which usually occur after the oxidative phase (Sánchez-Monedero et al. 2001).

The electrical conductivity usually increases throughout the composting process as a result of organic matter mineralization. However, although leaching losses were avoided (by isolating the piles from the ground and covering them with fabric), the electrical conductivity decreased (Fig. 1e). This can be attributed to the material used. Seaweed and fish waste contain more water and are more readily degraded than pine bark, so that the volume of the former decreased more than that of the latter. This produced a dilution effect throughout the process. Furthermore, many of the salts released by mineralization of the organic matter were absorbed by the pine bark, which is very porous and has a high absorbent capacity (USDA Forest Service 1971). Both of these processes lead to a gradual decrease in the EC of the mixture.

Two and a half months after the start of the process, 3 m3 of compost was produced by the decomposition of 10 m3 of raw material in each pile (i.e., there was a 70 % decrease in volume).

Characteristics of the final compost

Once the compost was obtained, the corresponding tests of stability, phytotoxicity, and safety were carried out. The results of these tests are shown in Table 2.

Table 2 Tests of stability, phytotoxicity, and safety
Full size table

In the Dewar flask or self-heating test, there was almost no difference between the temperature inside and outside of the Dewar flask, so that the degree of maturity was classified as V, which indicates that the compost was totally stable and that most of the biodegradable material had already been transformed (Brinton et al. 1995). The maturity was also established by measuring other parameters such as the DS of the organic matter and the nitrogen resistance (Soliva et al. 2004). The mean value of the DS was 51.4 %, which suggest that more than half of the TOM was ROM.

The germination index in the compost was higher than 95 % in all piles, which according to Zucconi et al. (1981) indicates the absence of phytotoxic substances or the presence of only very low levels.

As regards the safety of the compost, the results indicated that all three piles were free from Salmonella and helminth eggs and had very low levels of E. coli. The compost thus complies with the Spanish law concerning fertilizers (RD 824/2005 2005) and meets the requisites established by the EU ecolabeling scheme for soil amendments (EC Council 799/2006 2006) and plant growth substrates (EC Council 64/2006 2006). These pathogens were effectively reduced because the temperature of the compost remained higher than 55 °C for more than 30 days (US Environmental Protection Agency (USEPA) 1993).

Although compost has mainly been used in agriculture as a fertilizing agent and soil amendment, it is increasingly used as a substrate or as component of substrate because peat has a high price and above all it is a nonrenewable resource (Abad et al. 2001). The BD of the material used as a growth medium should be low, and the material should be highly porous with a good distribution of water and air (Abad et al. 1992; Noguera et al. 2003).

The values of the physical properties are shown in Table 3. There were no significant differences in the physical parameters between the three piles. The values of the TPS (mean value, 86.4 %), BD, and PD were suitable for use of the material as a substrate (Abad et al. 1992; Noguera et al. 2003) (Table 3). The water retention curve indicated a high capacity for aeration (45.8 %), but with an imbalance in the pore distribution as large and small pores predominated over intermediate-sized pores, which contain the EAW. Therefore, the compost contained only small amounts of EAW and buffering water, and high amounts of UW.

Table 3 Physical properties of the compost in each pile and overall mean value
Full size table

This type of water distribution may be valuable for horticultural or forest nurseries. In horticultural nurseries, small containers (6–7 cm high) are used and are usually watered little and often, so that a substrate with a high aeration capacity would help prevent problems related to anaerobiosis. The same also applies to the deep containers that are used in forest nurseries to enable the moisture to concentrate in the lower parts and thus encourage growth of deep roots.

The mean moisture content of the final compost was close to 40 %, with no significant difference between the piles (Table 4). This value is within the range required for use of compost as humic organic amendment (RD 824/2005 2005). This moisture content does not limit the use of compost for other purposes.

Table 4 Chemical and physicochemical properties of the compost in each pile, overall mean value and legal limits for different uses
Full size table

The pH was close to neutral and there were no differences between the piles. This pH ensures a good availability of nutrients for the plants and for biological activity. However, there were significant differences in the electrical conductivity between piles (Table 4); in all cases, the values exceed the limit permitted for growth substrates by the EU ecolabeling scheme (EC Council 64/2006 2006).

This high salinity, indicated by the high value of electrical conductivity, is mainly due to the fish waste (Table 1). Although the fish waste was mixed with pine bark, this was not sufficient to overcome the high salinity. Leaching of salts was minimized by not watering the piles and by isolating them with geotextile fabric. Soluble elements were present in the following order of abundance: Na+ > K+ > NH4 + > NO3 − > PO4 3− > Ca2+ > Mg2+. Although the contents of soluble elements did not generally differ significantly in three replicate samples of compost (Table 4), pile 3 contained lower levels than the other two piles.

In future trials, it would be interesting to try to lower the salinity of the fish waste, which contributes most to the overall salinity of the mixture. Some tests were carried out with samples of the fish waste, which were subjected to different treatments (leaching with different amounts of water for different times of contact and immersion). The most effective method was immersion of the fish waste in fresh water for 30 min, which lowered the electrical conductivity by almost 50 % (Fig. 2).

Fig. 2
figure 2

Changes in the salinity of the fish waste after immersion for different periods of time in fresh water in a proportion of fish/water of 1:1; error bars show the standard deviation of three replicates

Full size image

The organic matter content of the compost was almost 80 %, and therefore, the compost can be classified by the EU ecolabeling scheme as a soil amendment, OM ≥ 20 % (EC Council 799/2006 2006), and as an “organic compost amendment,” OM ≥ 35 % (RD 824/2005 2005) (Table 4). Moreover, piles 1 and 3 reached the optimum levels (OM > 80 %) cited by Abad et al. (1992) for use of a material as a plant substrate.

On average, the N content represented 2.1 % of the final product, with no differences between piles. In addition, the organic N represented more than 80 % of the total N. These values are within the levels permitted for soil amendments by the EU ecolabeling scheme (EC Council 799/2006 2006).

The C/N ratio differed significantly between the three piles, and the overall mean value was 21.9, which is considered optimal for plant substrates (20–40) according to Abad et al. (1992), although slightly higher than the 20 recommended by the Spanish legislation for use as an “organic compost amendment” (RD 824/2005 2005).

The proportion of N/P/K in the compost was 2.4:1.6:1, which indicates a good supply of macronutrients. The total contents of Ca and Na were approximately 1 %, with some differences between piles. The amounts of soluble, exchangeable, and total sodium in the compost were high, which prevents the use of compost in arid and semiarid regions with low rainfall. However, in humid areas, sodium is readily washed out and does not cause any problems for cultivating plants or as regards soil.

The exchange cation contents differed significantly in the different piles. In general, sodium was the most abundant cation, followed by calcium, with magnesium and potassium present in lower proportions. The mean CEC of the compost was 55.3 ± 1.8 cmol(+) kg−1, which is within the range considered optimal for use as a substrate with intermittent fertirrigation (Abad et al. 1992), as in this way, the cations are available for the plant and are not readily leached by watering.

The metal contents of all of the piles were very similar and enabled classification of the compost as type A, which is suitable for use in any type of cultivation system. The metal contents were much lower than the maximum values stipulated in the Spanish legislation concerning fertilizers (RD 824/2005 2005), substrates (RD 865/2010 2010) and in the criteria established for soil amendments and plant growth substrates by the EU ecolabeling scheme (EC Council 799/2006 2006; EC Council 64/2006 2006).

In conclusion, the co-composting of discarded fish, drift seaweed, and pine bark appears to be a useful method of utilizing these waste materials. Composting the materials for 10 weeks produced a stable, hygienic material which was rich in organic matter and nutrients and had a good structure and a low metal content. The material could be rendered suitable for ecological agricultural use as a fertilizer, soil amendment, or plant growth substrate by lowering the salinity.