2.5.1 Anaerobic ponds
These are the first ponds and are the recipients of the influent from homes and industries which is thick and dark in colour. These are responsible for anaerobic respiration so they are mainly concerned with the presence of anaerobic bacteria that digest the wastewater which is highly organic at this stage. They are the deepest of the ponds as anaerobic bacteria do not require oxygen and sunlight in order to digest. Mara (2004) concurred by suggesting that these ponds are usually deeper due to sludge accumulation and the main function is to remove biological oxygen demand in a relative short retention time of few days. Organic matter is removed by the sedimentation of settable solids and anaerobic digestion in the sludge layer. Theoretically, anaerobic systems should generate lesser amounts of sludge compared to aerobic systems, however, in practice in the Australian meat process industry, anaerobic ponds frequently fill rapidly with solids (Watson: 1999).
Anaerobic ponds are usually used for treatment of industrial and agricultural wastes which contain high organic matter and sulphate (Rhajbhandari: 2007). Anaerobic ponds provide a degree of pre-treatment, thereby enabling a reduction in the size requirements of the subsequent aerobic ponds. Anaerobic treatment is more suited to wastewater with high BOD (IETC-UNEP, 2002) and therefore useful at reducing high concentrations of BOD and suspended solids from agricultural and food processing wastewater.
A properly designed anaerobic pond can achieve around 60% BOD removal at 20° C and one-day hydraulic retention time is sufficient for wastewater with a BOD of up to 300 mg/l and temperatures higher than 20° C (Mara: 2003). The anaerobic ponds acts mostly like an uncovered tank that breaks down the organic matter in the effluent through the use of organisms, releasing methane and carbon dioxide( Quiroga: 2014).
The main operational problems of anaerobic ponds are the odour problems, mosquitoes and other insects and the possible reasons of odour problems are excessive loading rate, presence of toxic substances and inhibitors in influent, sudden drop of temperature and low influent pH value (Marinella: 2015). When considering the effects on climate change also, another disadvantage of anaerobic pond systems is the emission of greenhouse gases such as methane, carbon dioxide and nitrogen oxide that are normally released to the atmosphere since the areas are open and need sunlight and wind to operate (Glaz: 2016).
Diagram showing different types wastewater ponds namely anaerobic, facultative and maturation ponds
2.5.2 Facultative Ponds
These are the largest of the waste stabilisation ponds and they harbour both aerobic and anaerobic bacteria and Shelton (2005) concurred that they are referred as fulcatative ponds because the term facultative refers to the fact that these ponds operate with both aerobic and anaerobic zones. Reed (1995) stated that the facultative pond is the most common type used in the United States of America and in different terms such as oxidation pond, sewage lagoon and photosynthetic pond.
According to Palacios (2014), the waste water treated in the aerated ponds is discharged into the facultative ponds which need to fulfill two fundamental requirements of fulcatative ponds that are to have an adequate organic load and an oxygen balance that keeps the aerobic conditions over the anaerobic layer situated in the bottom of the pond. The presence of algae in the aerobic and facultative zones is essential for the successful performance of facultative ponds (EPA, 2002). In sunlight conditions, the algal cells utilise carbon dioxide from the water and release oxygen produced during photosynthesis. The oxygen produced by algae and surface reaeration is then used by aerobic and facultative bacteria to stabilise organic material in the upper layer of water.
Fulcatative ponds can be broadly classified as primary or secondary based on the characteristics of the influent. If the facultative pond receives influent without pre-treatment, it is named as primary facultative pond whereas if the fulcatative pond receives pre-treated influent from anaerobic pond, septic tank or shallow sewerage systems, it is called a secondary facultative pond (Sperling: 2007).
According to Environmental Protection Agency (2002), facultative ponds are usually 1.2 to 2.4 m in depth and are not mechanically mixed or aerated. The wastewater is more greenish and there is the presence of algae in water. They are designed for BOD removal on the basis of a relatively low surface loading to permit the development of a healthy algal population as the oxygen for BOD removal by the pond bacteria is mostly generated by algal photosynthesis (Mara and Pearson: 1998). The algae is beneficial to the process as it uses the carbon dioxide produced by aerobic bacteria to grow and release more oxygen which is needed by the aerobic bacteria for survival. This interrelationship between algae and aerobic bacteria is called symbiosis and allows for the removal of nutrients, heavy metals and pathogens (Alamgir: 2016).
2.5.3 Maturation Ponds
The effluent from fulcatative pond is channelled into the maturation ponds. The main purpose of maturation ponds is to remove pathogens found in the wastewater. These are very shallow, usually 0.9 – 1 m depth to allow light penetration to the bottom and aerobic conditions throughout the whole depth (Dehgani: 2007). The maturation ponds have a similar purpose than the facultative ponds with the difference being that in these ponds, there is hardly any accumulation of solids and the increase of the pH due to the photosynthetic activity results in an important bacterial mortality. Kayombo (2015) also added that maturation ponds usually show less vertical biological and physicochemical stratification and are well-oxygenated throughout the day.
The number and size of maturation ponds is defined by the necessary retention time required for the removal of faecal coliform and it should also be noted that the above also performs the oxidation of a small amount of biological oxygen demand (Martinez 2014). Total nitrogen removal in a whole waste stabilization system depends on the number of maturation ponds included in the waste stabilisation ponds system Pena (2004).
National centre for Groundwater defines groundwater as water that is found beneath the earth’s surface and it is an important source of drinking water especially in the rural areas (Rotatu, 2008). It is also fresh water from rain or melting snow and ice that soaks into the soil and is stored in the tiny pores between the rocks and particles of soil (EPA, 2018). The quality of groundwater is determined by various chemical constituents and their concentration, which are mostly derived from the geological data of the particular region through groundwater flows (Khumbar, 2011). Human activities can change the natural composition of groundwater through the disposal of chemicals and microbial matter on land or into the soils and this can lead to groundwater contamination which is the change of groundwater quality due to the activities of man (Harter, 2003).
The sources of groundwater contamination can be natural in nature whereby naturally occurring particulates of the soil such as iron, fluorides, manganese, arsenic, chlorides, or sulphates can become dissolved in ground water. Other naturally occurring substances, such as decaying organic matter, can move in ground water as particles (EPA, 2018). Man-made or human activities also promote groundwater contamination. Activities such as agricultural development, surface water irrigation, chemical use in agriculture, urban and industrial development affect the quality and quantity of groundwater as well ( USGS, 2016). In Jordan, Al Ramtha wastewater treatment plant was discovered to be the main cause of groundwater pollution and high levels of nitrates were found on nearby wells (Obeidat).