ABSTRACT
In this study, a Laboratory-scale Horizontal Subsurface Flow Constructed Wetland has been designed, and three replicas constructed, cultivated with selected locally available aquatic plants Phragmites australis,Polygonum salicifolium + Ipomoea carnea, respectively, with the third cell un-vegetated, and used to treat tannery effluent. The capacity or treatment efficiency or performance of the laboratory-scale Constructed Wetland in reducing the level of physicochemical parameters (Biological Oxygen Demand, Chemical Oxygen Demand, Nitrates, phosphates, suspended solids and chromium) under a 6day Hydraulic Retention Time was evaluated. Physicochemical parameters were analyzed using standard methods.Highest removal efficiency for BOD (97.9%), COD (94.2%),NO-3(54.4%), PO43-(44.1%) and Cr (98.4%) was observed in the wetland cell planted with Phragmites australis, while highest removal efficiency for TSS (92.6%) was observed in cell panted with a mix of Polygonum salicifolium + Ipomoea carnea. The un- planted cell showed lesser removal efficiency across all the selected parameters. Generally, through the 60 days of operation, the vegetated or planted cells showed better removal efficiency over the un-vegetated cell indicating that the selected aquatic plants are potential candidates for large-scale tannery wastewater treatment in Horizontal Subsurface Flow Constructed wetlands.However, evaluation of the system over longer period is required before concluding whether these plants in subsurface constructed wetland are efficient for primary treatment of tannery wastewater.
CHAPTER ONE
INTRODUCTION
1.1 Background of Study
The quantity of fresh water resources on earth is diminishing rapidly as a result of increasing World population andurbanization, especially in developing countries(Bruch et al.,2011). Already, many developing countries are facing acute scarcities of water due to rapid and unplanned development. In many developing countries, industrial effluents, sewage and municipal wastewaters are discharged directly into water bodies, and such indiscriminate discharges further intensify the shortage of water supply due to pollution of freshwater sources. In developing countries the situation has become more severe and critical because of lack of adequate sanitation. According to WHO (2009),1.5 million children die annually world-wide, from water-borne diseases, especially diarrhoea, as a result of lack of clean drinking water. The scarcity of safe drinking water is more severe in Africa and South Asia, where more than 80% of children deaths each year occur due to diarrhoea(WHO, 2008).
According to Mara (2004), wastewater is water supply after it has been used. Industrial activities produce large volumes of wastewater which is a major source of pollution to surface and ground water. Industrial pollution is one of the problems presently facing Nigeria. Effluents generated by the industries are the major sources of water pollution. Contaminated air,soil and water by effluents from the industries are associated with a heavy disease burden. Some heavy metals contained in these effluents (either in free form or adsorbed in the suspended solids) have been found to be carcinogenic (Tamburlini etal.,2002); while other chemicals equally present, are poisonous depending on the dose and exposure duration.These chemicals
are not only poisonous to humans, butar e also found to be toxic to aquatic life, and may result in food contamination (Novick, 1999). In addition to the above mentioned pollutants, plant nutrients such as phosphates and nitrates are pollutants because huge quantities enter the hydrosphere in runoffs from industrial effluents (Matsuo et al.,2001).The World Bank (1995)reported that about 19,000 tonnes of hazardous waste is produced annually in Nigeria and that the waste comes mainly fromsteel,metal processing, pharmaceuticals, textiles, tanneries, and oil refining industries.
Industrial processes generate a wide variety of pollutants. The characteristics and level of
pollutants vary from industry to industry (Davis, 2010).
Wastewaterfromleatherprocessingindustriesisverycomplexandleadstowaterpollution if discharged untreated, especially due to its high organic loading and chromium content (Tadesse, 2010). This uncontrolled discharge of tannery wastewater has caused the deterioration of water quality in many waterways (Calheiros et al., 2009) especially in countries like Nigeria where the leather industry is one of the oldest cottage industries.
The problem of environmental pollution received serious consideration only in recent years. The pollutants from tanneries have caused considerable damage to water courses, affecting drinking water supply, irrigation and aquatic life. The untreated tannery wastewater, gives rise to odour nuisance, unsightly appearance creating ground and surface
water pollution when untreated. Against this background,
pollution control is just unavoidable for the tanning industry in order to keep the environment clean and pollution free (Daryapurkar et al., 2005).
In Nigeria,the formal sector accounts for over 40 tanneries, while the informal sector accounts for a larger number of small and medium-scale tanneries. Most of the tanneries are located in the industrial estates of Kano, Sokoto, Plateau, Zaria, and Maiduguri. Many of the big tanneries
are mechanized and export-oriented especially those in Kano. Tanneries produce wastewater,
solid waste and gaseous emissions. Of these, wastewater is by far the most important environmental challenge due to tanneries world-wide(Sarkar, 1981).Tannery waste waters are characterized by high chemical oxygen demand (COD) and biochemical oxygen demand (BOD) because of the organic contents of the raw material (Wangetal.,2009). According to Oke et al., (2006),Nigeria is one of the main leather producers in West Africa accounting for about forty tanneries that process about 1, 860 tonnes of hides and skins per day. There is one leather institution in Nigeria (Zaria) with extension centers in (Kano, Sokoto, Maiduguri and Jos) plus the many small to medium scale tanneries in the country that cannot afford investments in pollution remediation equipment and technologies, because of their low profit margin (EPA,1997). They therefore discharge their wastewaters with hardly any form of treatment into the environment. As a result of this practice, thereis that grave need to enforce environmental protection measures and monitoring of tanneries in Nigeria.
The paucity of water resources and sustainable use of alternative water sources have led to demand for the development of wastewater treatment methods (Kaseva, 2003; Kyambadde, 2005; Doostietal., 2012). There are different conventional methods for wastewaters treatment such as Activated Sludge Process(ASP),Rotating Biological Contactor(RBC),Stabilization Ponds, Oxidation Ditch, Trickling Filter(TF),Sequence Batch Reactors (SBR), Aerated Lagoons and Up-
flow Anaerobic Sludge Blanket(UASB) and Micro-algae Techniques among others.Thesemethodshave limitations like energy needs,economic needs,need for large land areas,
generation (Simi and Mitchell, 1999;
Tanner and Sukias,2003; Sayadietal.,2011).Currently, the global interest in simple,safe,cost-
effective and green technology is paramount.
Constructed wetlands as a natural process,environment friendly, eco- friendlywith simple construction and low maintenanceis one of the techniques of interest (Vymazal,2002; Rousseau, 2008; Kadlec and Wallace,2009).
Constructed Wetlands (Subsurface Flow) are also referred to as rock-reed filters, vegetated submerged beds (VSB), reed beds and root-zone method amongst others (Crites and Tchobanoglous, 1998). The potential for Constructed Wetlands for wastewater treatment in developing countries has been described as enormous (Denny, 1997; Harberl, 1999; Kivaisi, 2001). This assessment was made on the basis of their low cost and ease of operation and maintenance when compared to conventional treatment systems, and that they represent an appropriate and sustainable technology for wastewater treatment - properties which have been widely documented.
The warm tropical and subtropical climates found in many developing countries are also ideal for productive biological systems such as Constructed Wetlands, particularly in small rural communities (Kivaisi, 2001; Rivera et al., 1995). And yet the uptake of this technology has been slow (Denny, 1997). According to Kivaisi (2001), Constructed Wetland systems have not found widespread use in developing countries, due to lack of awareness and local expertise in developing the technology on a local basis.
The potential of using plants for wastewater treatment has been intensively studied and has highly been examined all over the world, (Vymazal 2000; Vacca et al, 2005). One of the
treatment techniques which has been deeply examined is the Constructed Wetland (CW). The use of CWs is now recognized as an accepted low cost eco-technology, especially beneficial to small communities that cannot afford expensive conventional treatment systems (White, 1995; Billore et al., 1999).
Tanneries and Leather Science institutions can therefore use this technology to improve the quality of their effluents before discharge into surface waters.
1.2 Statement of Research Problem
The increasing number of tanneries in Northern Nigeria, coupled with a lack of technology for the efficient effluent treatment and lack of enforcement of National Environmental Standards and Regulations Enforcement Agency(NESREA)effluent standards and policies, have led to serious consequences for surface and ground waters receiving tannery effluents.
Wastewaters from leather processing are very complex, containing high organic loading and heavy metals, especially chromium, which is toxic to plants, animals and micro-organisms
evenat low concentrations. It disrupts the food chain, causing soil
salinity in irrigated farmlands and possibly affects aquatic organisms (Alves etal.,1993,Asaye,2009).
Also, the discharge of untreated tannery effluents leads to a gradual depletion of dissolved oxygen (DO) in soil and water. This also poses a serious threat to the lives of aquatic animals and micro-organisms.Consequently, human and animal lives are affected through direct or indirect consumption of polluted drinking water and polluted aquatic or land animals. Thus, discharged untreated into the environment, tannery effluents cause increased pressure on
1.3 Justification of the Study
The Leather industry is one of the oldest cottage industries in Nigeria and although tanning has been in existence for a long time, the problemof environmental pollution has received serious considerations only in recent years.
Many treatment methods that have been developed, end up failing due to the ambiguous
techniques involved and lack of proper operation, management, complex construction, sensitivity to temperature and excessive sludge production (Simi and Mitchell, 1999; Tanner and Sukias,2003;Sayadietal.,2011).Conventional high-technology wastewater treatment systems are, in many situations, not suitable in developing countries because they are not sustainable. They require uninterrupted power supply, replaceable spare parts and highly skilled labour for operation and maintenance (Konnerup et al., 2009). Another issue is the treatment requirement- In developed countries, the goal is the elimination of all pollutants like pathogens, nutrients, organic and inorganic chemicals (advanced wastewater treatment); whereas, the aim of treatment in developing countries is the reduction of pollutants to limits that can protect public health by controlling pathogens and preventing transmission of water borne diseases (Kivaisi, 2001). For these purposes, constructed wetlands are suitable since they can be efficient in removal of BOD, pathogens and nutrients(Konnerup et al., 2009).
The use of properly designed and constructed wetlands is the ideal alternative for treating tannery wastewater, giving that it is relatively cheap to establish, and requires little or no skills
in their operation and maintenance.Added benefits of constructed wetlands, according to the UNESCO-IHE (2006) are that, constructed wetland systems directly support millions of people by providing services and use to people for agriculture,in fishery, for timber;also CW reeds for mats making and thatch roofs. Direct use may also take the form of recreation, such as bird watching or scientific study.
1.4 Aim and Objectives
The aim of this study is to investigate the potential use and efficiency of a model constructed horizontal sub-surface flow wetland, in treating tannery wastewater.
The specific objectives of this research are to-
i. Design and construct a laboratory scale horizontal sub-surface flow wetland.
ii. Evaluate the potential of the aquatic plants Phragmites australis(common reed) and Polygonum salicifolium(slender knot weed) + Ipomoea carnea(morning glory) as suitable wetland plants.
iii. Assess the capacity of the model Constructed Wetland in reducing physicochemical parameters (Biological Oxygen Demand, Chemical Oxygen Demand, Nitrates, phosphates, suspended solids and chromium) from tannery wastewater obtained from The Nigerian Institute of Leather and Science Technology (NILEST) using Phragmites australis, ( common reed) Polygonum salicifolium (slender knot weed) and Ipomoea carnea(morning glory) as the aquatic plants in the systems.
iv. Evaluate the treated effluent against recommended National Environmental Standards and Regulations Enforcement Agency (NESREA) and World Health Organization (WHO)standards, and make valid recommendations dependent on the results of the study.
1.5 Scope of the Study
This study was restricted to a model horizontal sub-surface flow wetland. The tannery wastewater to be treated was collected fromThe Nigerian Institute of Leather and Science Technology(NILEST) and the aquatic plants for the wetlands are Phragmites australis, polygonum salicifolium and Ipomea carnea which were sourced locally from the banks of the Ahmadu Bello University dam. All the requirements for the construction and set-up of the wetland will be locally sourced in Zaria. The construction and set-up of the wetland was within the premises of The Department of Water Resources and Environmental Engineering, Faculty of Engineering, Ahmadu Bello University Zaria.
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