Use of Zeolite (Chabazite) Supplemented with Effective Microorganisms for Wastewater Mitigation of a Marine Fish Farm

By inergency On Feb 6, 2024

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1. Introduction

Over the past 35 years, marine aquaculture production has grown from one million tons per year to about 55 million tons [1]. Onshore and offshore fish farming certainly has a very critical aspect in the impact of farm wastes on coastal marine areas.

According to Pearson and Black [2], intensive aquaculture can have environmental impacts both on land and offshore. Phosphorus release is 19.6–22.4 kg/tons of product, of which 34–41% is released in dissolved form [3], while nitrogen dispersion is 52–95% of the nitrogen in the feed [4].

In seabream and seabass farm wastewater, ammonium nitrogen is more than 80% of the dissolved inorganic nitrogen, and depends mainly on the feed, pointing out the importance of adequate and proper nutrition [5]. Intensive land-based fish farms produce high particulate organic matter (food residues, feces, algal debris, etc.) that is discharged from wastewater and leaks into the final receiving environment. This process leads to turbidity, enrichment in nutrients and organic matter, alteration of sediment biogeochemical processes, disruption of benthic communities, and anoxic condition dominance. Wastewater from intensive aquaculture released into low water turnover environments, such as estuaries and lagoons, especially non-tidal lagoons, can exacerbate these issues.

The impact of aquaculture remains an unsolved problem to this day, which is addressed and continues to attract many researchers to this realm [6,7,8,9,10,11]. However, methods of reducing the nutrient load of wastewater from land-based fish farms, which move a large mass of water, are mainly based on the use of opportunistic nitrophilous macroalgal species. The management of these species presents actual difficulties and requires expensive efforts [7]. If the commercialization of algal masses and their processed products were viable, there would be an economic return to contribute to the costs of aquaculture wastewater mitigation, but this is rarely the case, especially in countries with high industrial development. Hence, there is a pressing need to explore low-cost alternative methods. Furthermore, the impact of eutrophication often requires costly downstream interventions in order to mitigate its consequences on the final receiving environment [12,13].

Zeolite has been used as an ion exchanger to absorb ammonium [14]. It was extensively tested in agriculture [15,16,17,18], freshwater basins [19], sewage effluent treatment for the reduction of ammonia and heavy metals [20,21,22,23], and more recently in the breeding of seabass juveniles [24]. In a seawater experiment, Lopez-Ruiz and Goméz-Garrudo [25] showed that more zeolite must be used to achieve the same amount of ammonium subtraction as obtained in freshwater experiences.

The types of zeolites vary in relation to the ionic ratios of the crystal structure between silicon, aluminum, and iron, e.g., clinoptilolite and mordenite have high Si/(Al + Fe) ratios, heulandite, chabazite, phillipsite, and erionite have intermediate ratio values, while analcime and laumontite have low values. The chelate ions within the voids of the crystal structure normally follow the following affinity order: K⁺ > NH₄⁺ > Na⁺ > Ca⁺⁺ > Mg⁺⁺. Therefore, it is the value of the ionic concentration that determines the metal cation substitution with NH₄⁺: it is the most abundant cation that occupies the chelation sites, in relation to affinity [26]. The NH₄⁺ removal consequently leads to a reduction of undissociated ammonia (NH₃) with which it is in a chemical equilibrium that depends on pH, T, and salinity [27]. Once the ion replacement pores are saturated by the NH₄⁺ ion, the action of the mineral against this ion ends; in essence, there is a saturation of the mineral [26].

Promoting bacterial processes of nitrification and denitrification is the most effective means of regeneration in the natural environment. Nitrifying bacteria oxidize NH₄⁺ to nitrate (NO₃⁻) [28] while denitrifying bacteria act on the nitrate by eliminating it from the system as N₂O and N₂ gaseous [29]. As soon as the flow of NH₄⁺ enters a system, the ion enters the bacterially acting biogeochemical cycle. If the load is high relative to the capacity of the bacteria to convert it, the presence of zeolite may be able to absorb the excess. When the NH₄⁺ concentration decreases, the chelated NH₄⁺ is released into the water, replaced by another cation, and can be converted by bacteria. New NH₄⁺ can then be trapped in the crystal structure, at the next flow. This process can occur spontaneously in the natural environment due to the bacterial pools in the sediment. Bacterial strains are found in the natural environment and could naturally form films around the zeolite crystals. Nevertheless, initial enrichment can expedite and enhance the process. Bacteria can be introduced to the treatment by mixing them with the zeolite crystals.

For semi-intensive aquaculture, small ponds, and wastewater from land-based rearing tanks, the nutrient load affecting ponds and tanks, as well as that conveyed with wastewater, could be mitigated through the use of mineral zeolite due to its high affinity for ammonium ion.

The aim of this study was precisely to verify the effectiveness of zeolite in reducing the nitrogen load in effluent from a marine land-based fish farm released into a non-tidal Mediterranean coastal lagoon. Looking ahead, the ultimate goal is to make these wastewaters more compatible with the lagoon environment.

3. Results and Discussion

Before the zeolite was placed on the T bottom (t0), the waters showed an increase in the variable values between input and output, in both T and B systems. Inorganic dissolved nitrogen (DIN as N-NO₃⁻ + N-NH₄⁺) increased from 281 ± 25 μM to 345 ± 60 μM (23%) in B, and from 305 ± 7 μM to 546 ± 195 μM (79%) in T. SRP increased from 2.75 ± 0.46 μM to 2.83 ± 0.17 μM (2.93%) in B, and from 2.70 ± 0.04 μM to 3.44 ± 0.43 μM (27.46%) in T. This suggests that the fish farm pond/canal system had evolved into a source of nutrients. In the surface sediment analysis conducted before treatment, TN was 0.551 ± 0.083% and TP 0.509 ± 0.091%, with an atomic ratio N:P of 2.4, highlighting a relative N-limitation [42]. This was likely a consequence of the different rates of accumulation of the two nutrients, with N being more easily released as ammonia and/or eliminated as gaseous N₂O and N₂ [43]. Settling basins can improve effluent quality, and significantly reduce N concentration in the water column [44,45,46,47]. However, it is crucial to manage to minimize nutrient release, which has accumulated mainly as debris organic matter [47].

Table 1 shows the means (±SD) in μM of the variables examined for the trials t2–t74, for both T and B systems. In Table 2, the increases or decreases between output and input from the system, for both T and B, are given by applying the formula: [(output − input) × 100/input].

The sample size in the t0 trial is low and not extended over time; nevertheless, notable trends emerge. T showed a decrease in nitrate nitrogen (−5%) and an increase in ammonia nitrogen (85%), while in subsequent trials, after the insertion of zeolite + EM, there was an increase in nitrate nitrogen and a decrease in ammonia nitrogen in the bottom water (Table 2). These observations suggest an enhancement in the nitrification process. At t0, B turned out to increase nitrate (6%) and keep ammonia nitrogen essentially stable (+1.1%). Later, between t2 and t74, B displayed stability in nitrate levels, but increased ammonium (Table 2). This suggests that the nitrification process likely remained inefficient overall in all trials. In t0, DIN increased in both B and T, while, in the subsequent trials, it increased even more in output from B, and decreased in output from T (Table 2). SRP values at the output of B exhibited minimal variation in trials t2–t74 (Table 2), compared with those at the beginning of t0, when there was an increase of about 3%. In contrast, in T, SRP went from an increase at the output of 27% in t0 to a decrease in subsequent trials of about 10% and 9% for top and bottom water, respectively (Table 2).

However, statistical analysis showed no significant differences between input and output from the two systems for any variable. Statistical significance was obtained by comparing the differences (Δ) between output and input, between the two systems B and T at different significance levels (5% or 10%). The results obtained using the Mann–Whitney U test are shown in Table 3, whereas in Figure 2, Figure 3, Figure 4 and Figure 5 the boxplots are reported. Specifically, significant differences were observed in system T versus B, in bottom water (ΔBb vs. ΔTb) where N-NO₃ increased (p = 0.05), and in top water (ΔBa vs. ΔTa) where both N-NH₄ (p = 0.07) and SRP (p = 0.06) decreased.

The increase in nitrate nitrogen at the bottom in T during the t2–t74 trials suggests that the bacterial nitrification process may have been more active compared to B, although this was matched by a significant decrease in ammonium nitrogen only in T top water. Considering that the water column averaged only 50 cm, the decrease in ammonia nitrogen could express partly the subtraction of the ion by zeolite and partly its oxidation to nitrate. Conversely, in B, where nitrate decreased and ammonium increased, though not significantly, it is very likely that nitrification and denitrification were never efficient. Confirmation of an enhanced oxidative state of the zeolite-containing sediment layer is provided by the decrease in SRP output from T, likely due to precipitation and blockage by iron and manganese oxides–hydroxides [48].

It can be assumed that in the treated area, the application of zeolite + EM resulted in a modest, barely detectable improvement in effluent quality. This improvement persisted throughout the experiment, signifying that zeolite saturation did not occur permanently and bacterial activity promoted regeneration of the mineral’s crystalline pores, allowing activation of an ionic pump. Furthermore, it is likely that the effect of zeolite was partially masked by the bottom nutrient releases, which were evident in t0 for both B and T and in subsequent trials for B.

Addressing the impact of wastewater from aquaculture practices is imperative, considering it is a bottleneck that hinders the adequate development of future fish production. Although in small plant systems, effluent mitigation solutions are feasible [49], solutions to date are energetically intensive and economically unviable for fish farms with a large volume of water and high fish production, as in the case of the farm in this study that uses over 45,000 m³ per day. For these farms, it is necessary to study systems with low economic and environmental impact. Contrary to the intended purpose, the pond/canal system, designed to settle suspended solids and enhance bacterial processes to break down organic detritus, droppings, and leftover feed, and to promote the processes of oxidation and N removal from the system, had proven ineffective, ultimately contributing to increase nutrient load in the effluent. The use of zeolite partly succeeds in counteracting this process. Effective sediment management is crucial to reduce anaerobic mineralization. Several options can address this issue, such as periodic removal of organic sludge accumulated on the bottom, or aerobic mineralization through frequent sediment resuspension in the water column [28,50]. This process, conducted along the settling basin system, makes orthophosphates insoluble, facilitates nitrification [50], and, consequently, promotes denitrification [29,51].

4. Conclusions

The results obtained in this field experiment in a small stretch of canal demonstrate, albeit subtly, mitigation of eutrophication components conveyed by wastewaters from a land-based fish farm that uses more than 45,000 m³ of marine water per day and produces about 500 tons of sea bass and sea bream annually. The placement of the zeolite bed in the studied canal section notably enhanced the oxidative state of sediments and facilitated orthophosphate sedimentation, nitrification, and ammonia nitrogen reduction, in comparison to the untreated parallel section of the canal. These results, albeit of modest magnitude, were maintained throughout the entire 74-day study period, suggesting that the added pool of microorganisms to the zeolite likely contributed to sustaining the active ammonium ion chelation capacity of the mineral.

For a more comprehensive exploration of zeolite + EM performance, wastewater should again be tested on a pristine surface or within a system where a periodic jet stream of air or water is applied to the zeolite bed. This approach can help to maintain a heightened oxidative condition and prevent the deposition of excess debris fallout on the mineral.

We believe that the results are encouraging and give hope for the possible use of zeolite in the aquaculture of marine species as well, allowing for greater sustainability of the impact of wastewater through low energy consumption management. But there is still much to be studied, e.g., how to arrange the zeolite by studying low labor use systems that allow more exchange surface between the wastewater and the mineral, and how to reduce the problem of detritus fallout on the zeolite layer with the least possible energy expenditure, considering the high detritus load in effluents of intensive aquaculture farms.

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