About 2 million tons of waste are generated annually after wastewater treatment in Russian Federation . The main component of such waste is activated sludge. Sludge from municipal and industrial wastewater treatment plants is a 4 hazard class. It is a conglomerate of microorganisms, a mineral component and organic substances . The dry residue of activated sludge consists of 70-90% organic and 10-30% inorganic matter. The content of organic carbon is more than 60%. All this makes sludge a valuable secondary resource. Sewage sludge having high content of organic matter, macro- and micro-nutrients, can be used as fertilizer/soil conditioner for food, vegetable crop and plants, which in most cases can be beneficially recycled. Biosolids/sewage sludge generally contain useful compounds of potential environmental value [3,4,5].
However, the main part of such sediments in Russia is stored in sludge pits and dumps. This creates challenges with waste disposal and the loss of the resources contained in them. The disposal of such wastes and the production of soil ameliorants is an urgent task, the solution of which will allow obtaining an economic and environmental effect [6, 7].
There are several ways to dispose the excess activated sludge: deposition on sludge pits, biological processing by anaerobic microorganisms in digesters, thermal methods (technological combustion or pyrolysis of sludge), composting, etc [8,9]. There are some options for obtaining a raw material resource for the production of organomineral fertilizers and substrates for growing plants. However, the practical use of activated sludge in this industry is no more than 6% . The only drawback in the use of sewage sludge on agricultural land is its pollutant load (including trace metals such us cadmium, lead, copper, zink, mercury, organic compounds and pathogens) [10, 11, 12, 13].
In this regard, we set the task to investigate some agrochemical indicators of dehydrated sewage sludge from the biological treatment of industrial and municipal wastewater of the Arkhangelsk Pulp and Paper Mill (APPM), one of the leading timber and chemical companies in Russia and Eastern Europe, in order to determine the possibility of use for substrates suitable for growing coniferous plants on them.
- Materials and Methods
The object of the study was dehydrated activated sludge from the treatment facilities of the APPM, where both industrial and municipal effluents of the city of Novodvinsk are cleaned. Activated sludge have been kept on sludge maps for at least 1 year, which underwent special sample preparation. Since it is planned to use peat for the preparation of substrates, we also studied the phytotoxicity of peat selected in the Konoshsky district of the Arkhangelsk region.
Distilled water was the control for the safety of the objects for plant germination, and the Knop solution was the control for the possibility of using them an ameliorant . The test objects were oats and watercress. Exposure time – 1 hour. The ratio of activated sludge : water = 1:10, which is the most convenient for filtration, has been experimentally determined. In Petri dishes with extracts, 25 seeds of the test culture were placed in 5 replications, they were kept in a thermostat at a temperature of 25°C. After 72 hours, quantitative and biometric indicators were determined , namely: seed germination energy (%) — the ratio of the number of germinated seeds to the total number of seeds, root length (cm), coleoptile/hypocotyl length (cm).
- Results and Discussion
All experiments were carried out on the basis of the Laboratory of Biogeochemical Research in the Northern (Arctic) Federal University named after M.V. Lomonosov. Figure 1 shows the average quantitative and biometric indicators of test cultures. The results were processed by generally accepted statistical methods in the SPSS program, the significance of differences in mean values was assessed according to the Kruskal-Wallis and Mann-Whitney criteria. Differences for all the given data on indicators were considered significant at p < 0.05.
Fig.1. Average quantitative indicators of germination of test crops
A – germination energy, B – root length, C – coleoptile/hypocotyl length
No statistically significant difference was found in the indicators of oat germination energy in the extracts and control. On average, it is 67%. When comparing such indicators as the length of the root and the length of the coleoptile / hypocotyl, a stimulating effect of the activated sludge extract is noted, comparable to the effect of the Knop solution. The growth rates in the peat extract are comparable with the data obtained for water.
For watercress, the average germination energy is 92% and also does not show a statistically significant difference in the studied extracts and control. However, there is a stimulating effect on the growth of watercress extracts of activated sludge. This is especially showed in an increase in the length of the root, where this indicator significantly exceeds the length of the root in the Knop solution. Similar to the data on oats, the indicators of plants in the peat extract are comparable with the data on water. For all the studied extracts, no phytotoxic effect on the germination of the studied test objects was found.
Thus, the extract of the dehydrated activated sludge from the treatment facilities of the Arkhangelsk Pulp and Paper Mill does not have a depressing effect on the germination of test objects — oat seeds and watercress, and according to the root length and coleoptile indicators, it has a stimulating effect comparable to or exceeding the effect of the Knop mixture, and can be recommended for continuing research on the effectiveness and safety of using dehydrated activated sludge from APPM in the manufacture of substrates for growing coniferous plants.
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