Yüzen akmatik bitki sistemlerinde azot giderim prensipleri

Abstract

ÖZET Bu çalışmanın amacı, yüzen bitki olarak bir su mercimeği türü olan Lemna minor'un kullanıldığı yüzen akuatik bitki sistemi ile evsel atıksuda azot formlarının giderirninin gözlenmesidir. Bu amaç için öncelikle akuatik sistemler ve bu amaç için kullanılan bitki türleri tanıtılmış, bu sistemlerin tasarım parametreleri ve bazı örnek çalışmalar anlatıldıktan sonra, akuatik sistemlerdeki azot döngüsü ve özellikle nitrifikasyon anlatılmıştır. Bu sistemlerde evsel atıksu arıtımında azot giderimi incelenmesini içeren deneysel giderim çalışmaları 175 gün sürmüştür. Bu çalışma kapsamında, bir su mercimeği türü olan Lemna minor içeren iki akuatik bitki arıtma havuzu kullanılmıştır. Bu iki havuz, 10 x 10 cm2' lik sekiz karesel bölmeden ibaret olup her birini yüzey alanı 800 cm2' dir. Havuzlar farklı su derinliklerinde işletilmiştir. 'A' Havuzu olarak isimlendirilen ilk havuzun etkili hacmi 9.6 litre ve 'B' havuzu olarak nitelendirlen ikinci havuzun etkili hacmi 6.4 litredir. Her iki havuz da 0.4 L/gün, 0.576 L/gün, 0.720 L/gün, 1.1 L/gün, 0.95 L/gün ve 1.98 L/gün'lük debilerde ; beş ayrı hidrolik bekletme suresinde işletilmiştir. Hidrolik bekletme süreleri 3.3 günden 23 gün arasında değişmektedir. Reaktörlerdeki sıcaklık, çevrelerindeki içi su dolu bir havuz ve kış aylarında bir ısıtıcı ve yaz aylarında da su devridaimi ile 20-25 °C civarında tutulmuştur. Her iki reaktörde de bütün hidrolik bekletme sürelerinde TKN, NH3-N ve NO3-N parametreleri ölçülmüş ve çalışmanın ikinci döneminde giriş noktasının başlangıcından itibaren farklı noktalarda aynı parametreler ölçülmüştür. Su mercimeği bitkisi, iklimsel koşullara ve atıksu karakterizasyonundaki değişimlere karşı oldukça yüksek bir tolerans kapasitesine sahiptir. Bununla birlikte, bu bitkilerin ya da mikroorganizmaların arıtma prosesinde ne derece rol oynadığı saptanamamıştır. Sistemlerin tasarımında, hidrolik yükleme oranının, hidrolik bekletme süresine göre daha anlamlı bir parametre olduğu sonucuna varılmıştır. Arıtımın büyük bir kısmı, sistemlerin ilk gözlerinde gerçekleşmektedir. Bu nedenle, daha yüksek verim elde edebilmek için sisteme ardışık girişler gerçekleştirlmesi önerilir. Bu sistemlerde, sık hasat koşullarında, yüksek oranda azot gideriminin gerçekleştiği gözlenmiştir.

SUMMARY The aim of this study is observing the removal of the nitrogen content of domestic wastewater by floating aquatic plant system in which Lemna minor, a member of duckweed species, as floating plant is used. On the way to this aim, firstly, aquatic systems and the species of plants used for treatment are introduced, and after designing parameters of these systems and some of case studies from literature are given, nitrogen cycle in aquatic systems and especially nitrification is explained. In the experimental removal studies, exploring of the efficiency on the nitrogen removal in the treatment of domestic wastewater of these systems is lasted for 175 days. Floating Aquatic Plant Systems Aquatic treatment systems can be defined as wastewater treatment systems which use submergent, emergent or rooted aquatic vascular plants and their microbial and algal epiphytes as the principal treatment mechanism in managed or unmanaged pond or wetland applications. Aquatic treatment systems have been used to remove BOD, nitrogen and phosphorus from wastewater. Ammonium is the predominant inorganic form of nitrogen in wastewater and usually the component of concern for removal. The principal application of Aquatic treatment systems has been with nitrogen and phosphorus control in subtropical environs. There is a renewed interest in the use of Aquatic treatment systems for nitrogen control for small communities world-wide. Natural treatment systems for wastewater management are differentiated from conventional systems based on the sources of energy that predominates in the two treatment categories. In conventional wastewater treatment systems, nonrenewable, fossil-fuel energies predominate in the treatment process. While conventional treatment relies largely on naturally occurring, biological pollutant transformations, these processes are typically enclosed in concrete, plastic, or steel basins and are powered by the addition of forced aeration, mechanical mixing, and/or a variety of chemicals. Because of the power intensity in conventional treatment systems, the physical space required for the biological transformation is reduced considerably compared to the area required for the same processes in the natural environment. Natural treatment systems require the same amount of energy input for every kilogram of pollutant that is degraded as conventional biological treatment systems; xivhowever, the source of this energy is different in natural systems. Natural treatment systems rely on renewable, naturally occurring energies, including solar radiation; the kinetic energy of wind; the chemical-free energy of rainwater, surface water, and ground-water; and storage of potential energy in biomass and soils. Natural treatment systems are land intensive, while conventional treatment systems are energy intensive. Natural treatment systems are economical during both construction and operation stage. Therefore, they become more interesting especially for treated water can be used in agricultural irrigation. Floating aquatic plant systems are formed of one or more shallow ponds, in which water hyacinth, duckweed, or any other aquatic plant can grow up. Duckweed is among the smallest and simplest flowering plants and has one of the fastest reproduction rates. A small cell in the frond divides and produces a new frond, and each frond can reproduce at least 10 to 20 times during its life cycle. Lemna sp. Grown in wastewater effluent doubles in frond numbers and, therefore, doubles the area covered, every 4 days. Duckweed may grow at least twice as fast as other vascular plants. It is more cold tolerant than water hyacinth and is found throughout the world. A minimum temperature of 7oC has been suggested as the practical limit for its growth. Under freezing conditions, duckweed will lay dormant on the pond bottom until warmer conditions return. Treatment occurs in floating aquatic plant systems through three primary mechanisms: (1) metabolism by a mixture of facultative microbes on the plant roots suspended in the water column and in the detritus ant the pond bottom, (2) sedimentation of wastewater solids and of internally produced biomass (dead plants and microbes), and (3) incorporation of nutrients in living plants and subsequent harvest. Floating aquatic plant systems are typically effective at reducing concentrations of biochemical oxygen demand and total suspended solids. Nitrate nitrogen may be effectively removed by denitrification. Total nitrogen and phosphorus removal can be consistently accomplished if the plants are harvested routinely. Ammonium is the predominant inorganic form of nitrogen in wastewater and usually the component of concern for removal. The principal application of Aquatic treatment systems has been with nitrogen and phosphorus control in subtropical environs. There is a renewed interest in the use of Aquatic treatment systems for nitrogen control for small communities world-wide. Nitrogen removal occurs by a number of mechanisms: (I) plant uptake of ammonium or nitrate and subsequent harvesting; (ii) volatilization of ammonia; (iii) detrital settling of particulate nitrogen; and (iv) mircobial nitrification coupled with denitrification. Recent studies have shown the relative importance of these mechanisms in subtropical systems. Harvesting of excess biomass to maintain high productivity tends to increase the importance of this mechanism.. Settled material must be periodically removed from the system to prevent nitrogen mobilization from the sediments. xvIn summary, wetland treatment systems consistently reduce total nitrogen concentrations in many wastewaters. The magnitude of these reductions depends on many factors including inflow concentrations, chemical form of the nitrogen, water temperature, pH, alkalinity, organic carbon, dissolved oxygen, water depth, and biota. Although these factors can be incorporated with some success into design of wetland treatment systems, precise nitrogen reaction rates and performance under different environmental variable are not known. These observations will lead the engineer to design conservatively. MATERIALS AND METHOD In the content of this study, pilot scale aquatic plant treatment systems analyses have been carried out in laboratory conditions in order to research the removal of nitrogen forms from the domestic wastewater. Two aquatic plant treatment ponds are used in which Lemna minor, a species of duckweed has grown. The ponds are formed of eight square compartmens of 10 x 10 cm2 and the total surface area of the ponds are 800cm2. Each of the ponds are operated with different water heights. The effective volume of the first pond 'A' is 9.6 liters and the second Pond 'B' is 6.4 liters. Both two ponds are are operated in five different hydraulic retention time with the flow rates of 0.4 L/day, 0.576 L / day, 0.720 L/day,1.1.L /day, 0.95 L / day and 1.98 L/day.. Hydraulic retention times ranged from 3.3 days to 23 days. Temperature in the reactors are hold around 20- 25 °C and light has been supplied by a special lamp intensity of 2150 lux during day times; at night the lamp has been switched off by a timer. The characteristics of wastewater used in the experiments is given in Table 1. Table 1. The Characteristics of Domestic Wastewater Used in Experiments In both reactors TKN, NH3-N and NO3 -N parameters have been measured for all hydraulic retention times and in the second part of the study the same parameters are measured for different distances from the influent point. The Methods for analyses are given in Table 2. xviTable 2. The monitoring and the Analyses Program EXPERIMENTAL RESULTS The experimental study was carried out for 175 days. The results achieved by experiments are shown in the figures below. Reactor 'A' - Gh -Effluent Parameters at Steady State Conditions C(NH3-N),mg/L 0.8 10.4 Gh, days 18 23 -TKN,mg/L -N03-N,mg/L -NH3-N Figure 1. Reactor 'A'- 0h Effluent Parameters at Steady State Conditions xvuReactor'B' -8h- Effluent parameters at steady state conditions C(NH3-N),mg/L -TKN.mg'L -N03-N,mg/L -NH3-N,mg/L Figure 2. Rector 'B' - 6h Effluent Parameters at Steady State Conditions Variation of The Influent and Effluent Parameters according to 0h For Reactor' A' Influent Influent Influent influent Influent 36 j 33 a 24 £ 21 ?8 18 g 15 2 9 « 6 o- 3 0 Effluent Effluent İ nE 1 I Effluent I nl Effludnt I nE Effluent 10.4 18 6 h, days 23 ? TKN SNH3-N 0NO3-N Figure 3. Variation of the Influent and Effluent Parameters according tp 8h for Reactor 'A' The Variation of the Influent and Effluent Parameters According to 0h for Reactor 'B' Influent Influent Influent Influent Influent 35 30 + 25 - t» 15-. 10- 5-J- 0 Effluent nE Effluent J3E Effluent -+ l>» f- CM CM tO <0 V *`. T- *- 6h, days DTKN BNH3-N HN03-N Figure 4. Variation of the Influent and Effluent Parameters according to 0h for Reactor 'B' xvuiThe parameters changing according to distance from Influent Point For Reactor' A* (8h=10.4 Days) 30 60 Distance, cm -TKN -N03-N -NH3-N Figure 5. The parameters changing according to distance from influent point for Reactor 'A' (8h= 10.4 days) The parameters changing according to distance from Influent Point For Reactor' A' (6h= 5 Days) 20 â15 S 10 ü 5 30 60 Distance, cm -»-TKN -*- N03-N -.- NH3-N Figure 6.The parameters changing according to distance from influent point for Reactor 'A' (0h = 5 days) The parameters changing according to distance from Influent Point For Reactor'B' (8h= 7 Days) 12 10 8 8 6 4-; 4 - 2 0 -»-TKN -A- N03-N -.- NH3-N Distance, cm Figure 7. The parameters changing according to distance from influent point for Reactor 'B' (9h = 7 days) XIXThe parameters changing according to distance from Influent Point For Reactor'B' (8h=3.3 Days) 25 20 15 10 5 0 ifi 1 0.8 -TKN 06 J? ^^*~ N03`N 0.4 5 !-.- NH3-N 5. t 0.2 U -0 10 30 60 Distance, cm 80 Figure 8. The parameters changing according to distance from influent point for Reactor 'B' (9h = 3.3 days) CONCLUSION AND DISCUSSION The plant of Duckweed has a high capacity of tolerance for climatic conditions and wastewater characterization.. However, it is unknown how much that plants or the microorganisms take a role during treatment process. In designing the systems hydrolic loading rate is much more meaningful parameter than hydrolic detention time. A great major of the treatment is achieved in the first compartments of the systems. Hence, it is recommended that sequential influents must be provided for higher performances. In the conditions of frequent vegetations, a high rate of nitrogen removal is taken place in such systems. xx