As it emerges from the isotherm model analysis, the NO3-N adsorption on Sep, T-Sep, H-Sep and H-Pal is monolayered and fits on linear Langmuir isotherm; on T-Pal is monolayered as well but fits on nonlinear Langmuir model, while the interactions between NO3-N and Pal are heterogeneous. That renders the adsorption mostly a physical procedure with no strong bonding, even after modification, except from Pal sample where chemical bonds can be developed potentially at its inner space. However, after Pal modifications the interactions are changed as was expected, sine the modifications are mostly surficial. The linear and nonlinear forms of pseudo-first and pseudo-second order kinetic models, as well as the intraparticle diffusion kinetic model were applied on raw and modified Pal and Sep samples, in order to determine adsorbents surficial characteristics and insights in adsorption mechanism . According to the parameters of both linear and nonlinear models , all the examined models presented a good fit at all the samples. Specifically, at the linear forms of the models, the correlation coefficient of the pseudo second order expresses a better fit of the particular model on all the samples instead of the pseudo-first order model. Nevertheless, at the nonlinear forms, the parameters of pseudo-first order kinetic outweighs pseudo-second, mostly due to the agreement of qeexp variables with the qecaland low statistical SSE values as well, except from raw Pal . On the contrary, the qecal of linear pseudo-second order model are not representative of the qeexp of the treated samples, despite that the qecal of raw Sep can be correlated with the qeexp but higher SSE values were obtained as well.

Summarizing, the nonlinear pseudo-first order kinetic model obtained better agreement of qecal with qeexp and the lowest SSE values, expressing better the NO3-N adsorption on the examined samples, whereas the linear pseudo-second order kinetic model sufficiently approaches the adsorption procedure on palygorskite. In specific,nft system the pseudo-first and pseudo second order kinetic models assume that the adsorbate is getting adsorbed at a single surface and multiple surfaces at time t respectively . The results come in agreement with the isotherm models fitting, since the NO3-N adsorption on Sep, T-Pal, T-Sep, H-Pal and H-Sep was characterized monolayered but heterogeneous for Pal. Concerning the intraparticle diffusion model, the obtained parameters can be shown in Table 5. In all cases two different stages characterize the adsorption process, starting with the diffusion of the NO3-N at solids interfaces and then the diffusion at the inner pores .On the other hand, at Sep, T-Sep and H-Sep the Stage 1 kid > Stage 2 kid, thus indicating the rapid diffusion of NO3-N at the solids interface, compared to a very slow rate diffusion in their inner pores . The fast growth in world population has led to the intensification of activities like food production, energy generation and industrialization. In Sub-Saharan Africa alone, population increase is expected to double, whilst the world population is expected to reach 10 billion by the year 2050 . To meet this increasing populations’ food and energy demands, there has been an increase in intensification of agriculture and industrialization. This intensification has however resulted in overproduction of waste substances such as cow manure, resulting in inappropriate and untimed disposal of the manures especially in agricultural fields . Such disposal carry environmental challenges such as introduction of harmful trace metals, inorganic salts and pathogens into the soil . Technologies that can enhance the sustainable utilization of the large quantities of animal manures being generated throughout the world are currently being promoted.

One such technology that is gaining momentum due to its limited environmental footprint is the generation of clean energy using the animal manures in the form of biogas . The management of animal manures through biogas production is being widely promoted as an important renewable energy source . However, the sustainability of biogas production is reliant on suitable utilization of digestate from the biogas production, with their utilization as a fertilizer being the most plausible option.Several researchers have looked at the possibility of making biogas generation a clean energy and zero emissions process by evaluating the fertilizer potential of the effluent from biogas digestion. Studies have reported that the digestate from biogas are high quality nutrient materials rich in macro nutrients such as P, K and in particular, nitrogen.Due to the abundance of organic matter, nutrients and the presence of bio-active substances, biogas digestates are often applied directly to the soil as an organic fertilizer together with mineral fertilizers . However, the response of plants fertilized with biogas digestates is variable depending on manure type, with long term soil application presenting a challenge of heavy metal accumulation in soils . Due to the nutrient composition of biogas digestates, the liquid fraction has been evaluated as a potential fertilizer in soils and hydroponic systems. However the major challenge is their potential phytotoxicity, heavy metal and pathogen addition to the soil and plants . A study by Krishnasamy et al. evaluated the potential of diluted biogas digestates as a nutrient solution for silver beet under a hydroponic system. It was observed that silverbeet survival was best at 20% digestate whilst at 50%, silver beet survival was negatively affected due to ammonia toxicity and low oxygen . Lencioni et al. evaluated the phytotoxicity of pig digestate, which had been diluted from 5-30% on different plants, reporting that digestate concentrations that stimulated germination and early seedling growth were as low as 2–3%, whilst 20–30% could be used for the advanced stages with limited negative effects.

Tomatoes grown under a bato bucket hydroponics system supplied with nutrients made from a liquid effluent from anaerobic thermophilic digestion of poultry litter grew slowly and produced fewer and smaller tomatoes . This was attributed to the sensitivity of tomatoes to ammonia present in the nutrient solution at higher concentrations. However in another study where biogas slurry was used to fertilize tomatoes planted in soil, positive growth and nutrient uptake was reported . Though the few studies highlighted above have evaluated the fertilizer value of biogas digestates in hydroponics, there is paucity of information on research that has evaluated their effects on plant growth, nutrient uptake and crop yields. Furthermore, most of these studies have only focused on early seedling growth of leafy vegetables, with none of these studies evaluating their potential on the growth and yield of fruit based vegetables like tomatoes. Our study thus evaluated the phytotoxicity and nutrient potential of biogas digestates produced from cow manure obtained from the Namibian desert area, under a deep water culture hydroponic system for tomato production.This study was undertaken at the Sam Nujoma Campus of the University of Namibia, in Henties Bay. The digestate used in this study was collected from a biogas digester located at the Sam Nujoma Campus of the University of Namibia, which had been using cow manure collected from the Uis area located at the edge of the Namib Desert. The digester had been fed once off with fresh cow manure, and the digester room was maintained at a temperature of 30 Celsius throughout the biogas generation process. After exhaustion of the biogas generating process, the liquid fraction of the digestates were collected and stored in plastic containers, until the study was undertaken. Before the start of the study, a sample of the original digestate as well as those from the different treatments including the control were collected, filtered using Whatman No. 2 filter paper, then analyzed for cations and heavy metals using an ICP-OES .

The concentrations of inorganic P and N were analyzed using colorimetric methods outlined by Okalebo et al. . The pH and electrical conductivity was measured potentiometrically in the different solutions corresponding to the different treatments. The selected chemical characteristics of the digestates and control commercial nutrient solution used in this study are shown in Tables 1 and 2.Prior to establishment of the hydroponic study, a phytotoxicity test was undertaken to enable for the identification of the appropriate biogas digestate dilution ratio. In this test, the biogas digestate was diluted on a volume to volume basis, with deionized water to give the following treatments: undiluted biogas digestate; 10%; 20% and 40% biogas digestate, with the control being the normal hydroponic fertilizer. Based on the five treatments, the phytotoxicity test was undertaken using the seed germination and plant growth bioassays as described by Tiquia and Tam and Ravindran and Mnkeni.Briefly, the filtered sub-samples from the different treatment combinations were used to saturate two Whatman filter papers, which were then placed inside a sterile petri dish. Ten seeds of tomato , spinach , carrot , beetroot and cabbage were placed on top of the filter papers and incubated for 3 days in dark conditions at 25 Celsius. After the 3 day incubation, the seed germination, relative seed germination calculated based on Eq. , relative root elongation calculated based on Eq. and germination index calculated based on Eq. .Based on results of the phytotoxicity study and the elemental composition, the most appropriate dilution level for the biogas digestate of 10% was selected for use in the tomato crop growth study. Due to the low nutrient content in the biogas digestate, the treatments in this study were thus based on the various levels of commercial hydroponic gutter fertilizer substitution into the 10% diluted solution. This gave the following treatments: 100% normal hydroponic fertilizer ; 20%; 40% and 60% added as normal hydroponic fertilizer to supplement the 10% biogas digestate solution. These substitution levels were designed to allow the study to identify which level of the commercial fertilizer supplementation can give the highest crop growth whilst reducing the quantity of the expensive commercial fertilizer used.

The treatments based on the level of commercial fertilizer supplementation were replicated 3 times and laid in a completely randomized design. Rectangular plastic containers with a 30 L capacity were used for the deep water culture hydroponic system, with a floating styrofoam, onto where the hydroponic planting cups with the seedlings were placed. In each hydroponic planting tray, 2 tomato plants , were planted and these were monitored for pests and diseases until maturity. At maturity, the number of fruits, number of flowers, chlorophyll content, fruit fresh weight and fruit sugar content were measured. To determine the plant nutrient uptake, the tomato above ground biomass was harvested, dried at 60 Celsius and grinded with a plant grinder. The grinded plant samples from the respective treatments were then digested using a mixture of Nitric acid and Perchloric acid on a block digester as outlined by AgiLASA . The digested samples were then analyzed for total concentrations of Ca, K, Mg, Na, Pb, Zn, Cr and Cd using ICP-OES model iCAP 6000 Series; Thermo Fisher Scientific.The phytotoxicity of the various biogas dilutions were determined using seeds of five popular vegetables. significant differences were observed on relative seed germination for all vegetables except for spinach . The highest concentration of the biogas slurry i.e. the 40% treatment, resulted in significantly the lowest RSG in all vegetables except for spinach. At this highest concentration, the germination of carrot was completely suppressed resulting in a RSG of 0%. For most of the vegetables, the treatments with 10% biogas slurry resulted in the highest RSG which was higher than the control treatment of hydroponic fertilizer alone. The relative root elongation , which express the root growth relative to the control was generally highest in treatments where biogas slurry was added compared to the control . Across all treatments and vegetables, significant differences were observed on RRE. Generally, the RRE followed the order 10% biogas slurry >20% biogas slurry > control >40% biogas slurry. Across all the five crops, the 10% biogas slurry treatment showed an average of 116.5% RRE, which was 46.5% more than the control and 190.4% more than the 40% biogas slurry treatment. For all the crops except for beetroot, the 10% biogas slurry treatment resulted in the highest germination index . Similar to other parameters, the 40% biogas slurry treatment showed the lowest germination index for all crops except in spinach.