2143 Advances in Environmental Biology, 6(7): 2143-2150, 2012 ISSN 1995-0756 This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLE Corresponding Author Amin Mojiri, School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia E-mail: amin.mojiri@gmail.com A Review on Anaerobic Digestion, Bio-reactor and Nitrogen Removal from Wastewater and Landfill Leachate by Bio-reactor 1 Amin Mojiri, 1 Hamidi Abdul Aziz, 1 Nastaein Qamaruz Zaman and 2 Shuokr Qarani Aziz 1 School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia 2 Department of Civil Engineering, College of Engineering, University of Salahaddin–Erbil, Iraq Amin Mojiri, Hamidi Abdul Aziz, Nastaein Qamaruz Zaman and Shuokr Qarani Aziz; A Review on Anaerobic Digestion, Bio-reactor and Nitrogen Removal from Wastewater and Landfill Leachate by Bio-reactor ABSTRACT The nitrogen removal from wastewater and landfill leachate has become a vital part of the overall treatment process because nitrogen compounds have the significant impact on the environment. Generally, biological nitrogen removal is used for nitrogen elimination from wastewater. In the last years, newly advanced anaerobic reactor systems such as up flow anaerobic sludge blanket (UASB), anaerobic filter (AF), anaerobic fluidized bed (FB), anaerobic sequencing batch reactor (AnSBR) and other anaerobic reactors have been used for the treatment of low strength wastewater, landfill leachate and, etc. The aims of this study were the review on the anaerobic digestion, some kinds of bio-reactors and nitrogen removal from wastewater and landfill leachate by bio-reactor. Many investigations showed that the bio-reactor and anaerobic treatments could be reducing nitrogen from wastewater and landfill leachate. Additionally, many studies also were reported some environmental factors such as pH, temperature, ammonia, alkalinity and nutrients affecting on anaerobic digestion. Key words: Anaerobic digestion, bio-reactor, nitrogen removal, wastewater, landfill leachate Introduction One of the elements of concern in wastewater is nitrogen, especially since the use of synthetic nitrogen fertilizers produced from atmospheric N 2 by the Haber-Bosch process has increased tenfold over the last 40 years. The human contribution to nitrogen pollutions, especially in the form of urine, is ever increasing in view of the growing world populations. Discharge of this nitrogen into the natural waters can lead to eutrophication and oxygen depletion [11]. The nitrogen removal from wastewater has become a vital part of the overall treatment process because nitrogen compounds have significant impacts on the environment. Conventionally, removal of nitrogen is performed by means of a two step process. First, NH 4 + is oxidized to nitrite (NO 2 - ) or nitrate (NO 3 - ) by an autotrophic nitrification process, and this is subsequently reduced to nitrogen gas by a heterotrophic nitrification process. However, some wastewaters are rich in NH 4 + but poor in biodegradable organic carbon; these wastewaters are less suitable for biological nitrogen removal via the conventional nitrification-denitrification processes [18]. One of the common and economical methods in wastewater treatment is biological processes. The main objective of biological treatment is to convert organic materials into other products using microorganisms. Generally, biological nitrogen removal is used for nitrogen elimination from wastewaters. Although NO 2 - or NO 3 - can be present, ammonia (NH 3 -N) is abundant in many wastewater streams. NH 3 -N removal is often achieved using nitrification/denitrification processes. In such systems, nitrifying bacteria oxidize NH 3 -N to NO 3 - under oxic conditions, and NO 3 - is subsequently or simultaneously reduced to nitrogen gas under anoxic conditions [20]. Nitrogen removal is normally realized by sequentially alternating between oxic and anoxic situations or by the creation of separated zones with suitable conditions for nitrification and denitrification, respectively. Alternatively, high rates of simultaneous nitrification and denitrification (SND) can be achieved, in activated sludge and biofilm type systems alike, at operational conditions where both oxic and anoxic micro-environments are present. Nitrification can occur at the liquid/biomass