Anaerobic Reactor Design Concepts for the Treatment of Domestic Wastewater
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Reviews in Environmental Science and Bio/Technology (2006) 5:21–38 Ó Springer 2006 DOI 10.1007/s11157-005-4888-y Review Anaerobic reactor design concepts for the treatment of domestic wastewater Adrianus van Haandel1, Mario T. Kato2,*, Paula F. F. Cavalcanti1 & Lourdinha Florencio2 1Department of Civil Engineering, Federal University of Campina Grande, Rua Aprı´gio Veloso 882, 58109- 970, Campina Grande PB, Brazil; 2Department of Civil Engineering, Federal University of Pernambuco, Avenida Acadeˆmico He´lio Ramos s/n, Cidade Universita´ria, 50740-530, Recife PE, Brazil (*author for correspondence: phone: +55-81-2126-8228/8742; fax: + 55 81 2126-8219/8205; e-mail: [email protected]) Key words: anaerobic reactor concepts, classic reactors, combined reactors, design, domestic sewage, modern reactor configurations, performance, UASB Abstract Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented. 1. Historical evolution of anaerobic treatment 40%. After the development of the first aerobic systems in the decade 1910–1920, another mis- When the first anaerobic treatment systems conception was that the lower efficiency of the were developed by the end of the 19th century, anaerobic systems was attributable to a lower the design was not really adequate for good metabolic capacity of anaerobic bacteria and performance, possibly due to the misconception therefore a long retention time would be neces- that the settleable solids were the most impor- sary. This has confused sanitary engineers for a tant sewage component to be removed (McCarty long time and to a large extent persists until 1982). Although they are the most visible con- today. In reality, the excellent performance of stituent, they account for only about 1/3 of the the aerobic process like activate sludge was organic load in raw sewage, whereas another 1/ mainly due to the intense contact of the active 3 is present in the form of colloids and the bacteria and the influent organic material, guar- remaining 1/3 as dissolved matter. Much of the anteed by intense mixing by aeration. By con- colloidal and soluble fractions were not re- trast in classic anaerobic systems, mixing was moved because the design of the treatment sys- unintentional. tems was such that there was no possibility of The high potential of anaerobic systems was contact between the non-settleable fractions and shown by Young and McCarty (1969), who the bacteria that were active in the biological successfully operated an upflow anaerobic filter, reactor. As a result the organic material re- treating concentrated mainly soluble rum distillery moval efficiency was low, of the order of 30 to wastewater. Two very important improvements 22 were made: (i) the influent introduced at the bot- 2. Classic anaerobic treatment systems tom, following an ascending pathway, thus guar- anteeing intense contact between the bacterial Classic anaerobic sewage treatment systems are mass and the incoming organic material and (ii) related with the earlier digesters developed by the active sludge mass increased by introducing a Mouras in France (1872), Cameron and Travis in sludge retention device: a bed of macroscopic bod- England (1896 and 1903, respectively), and ies (stones) in the reactor, onto which the bacterial Imhoff in Germany (1906). They conceived tanks mass could adhere and grow. The improvements that are known as decanter-digesters, in which resulted in a digestion capacity of organic mate- settleable solids were retained and digested at the rial, in terms of chemical oxygen demand (COD), bottom by the anaerobic sludge. Typical of these of more than 10 kg m)3 day)1, which had not classic systems (septic tank and Imhoff tank) is been possible even with the most advanced aerobic the horizontal sewage flow through the system in systems. Thus, the poor performance of the early the upper part, while the anaerobic sludge rests systems was not because of the ‘‘weak’’ anaerobic at the bottom of the tank. Consequently, no spe- biomass, but because of the poor design. The cific effort is made to ensure and enhance the anaerobic filter today is still widely applied in sew- contact between influent organic material and the age treatment and considered as a precursor of the biological sludge at the bottom. These two classic new modern high rate systems developed in sub- systems found wide application until the decade sequent decades. The basis for the anaerobic treat- of 1930, when they started to loose place in fa- ment of high capacity with high efficiency was vour of aerobic sewage treatment systems, like established by the provision of the two essential the trickling filter and the activated sludge pro- requisites: intense contact and large bacterial mass cess. The maximum efficiency in the classic retention. anaerobic systems does not exceed 30 to 50% of An important breakthrough was the develop- the biodegradable matter, depending on the nat- ment of the upflow anaerobic sludge blanket ure of the sewage and the settling efficiency. (UASB) reactor by Lettinga and co-workers Nevertheless, both systems are still applied today (Lettinga et al. 1980). Originally developed for in many countries, especially for on-site treat- the treatment of concentrated (agro) industrial ment of sewage of individual dwellings or for wastes, soon its potential for sewage treatment small communities. Another flow-through reactor became apparent, especially in regions with a hot with horizontal flow is the anaerobic pond, climate. Hundreds of UASB reactors are now in which is amply used for communal sewage treat- operation in many countries, especially in regions ment, normally associated with other units of with hot climates. waste stabilization pond (WSP) systems. Although by now it has been amply shown that anaerobic systems, when properly designed, 2.1. Septic tank (ST) have a high capacity and efficiency, a single anaerobic reactor alone often cannot produce an Septic tanks are treatment units where primary effluent compatible with legal norms and regula- treatment (solids settling) is coupled to anaerobic tions. One alternative is to combine units in or- digestion of the settled sludge. A septic tank may der to have a pre-treatment step mostly to be constructed either as a single unit or divided remove solids, and subsequent anaerobic diges- into two parts: an upper chamber for settling tion of the soluble organic material in a high rate and a lower for digestion. The latter configura- reactor with an active anaerobic sludge. However tion (Figure 1) is called the Imhoff tank after its in many cases, it is still necessary to introduce a inventor. The advantage of separating the two post-treatment system in order to adapt the functions is that the generated biogas bubbles anaerobic effluents to the required discharge cannot hinder the settling of the solids. For that standards, in terms of residual organic matter, reason the liquid retention time in single cham- pathogenic microorganisms and possibly nutri- ber units (12 to 24 h) is longer than in the ents. Post-treatment of anaerobic effluents with Imhoff tank (2 h in the settling section). The aerobic or physical-chemical options are dis- shorter retention time is a considerable advan- cussed in a next chapter. tage, since the volume is reduced in the same 23 Figure 1. Schematic drawings of septic tanks with (a) single chamber, left; and (b) two chambers (Imhoff tank), right. Measure- ments are in meters. proportion. The treatment efficiency of organic the tank, decreasing the liquid retention time and material and suspended solids in raw sewage gen- the settleable solids removal efficiency. The accu- erally is of the order of 30 to 50%. mulated digested sludge must be removed period- Because of their simplicity and low cost, sep- ically from the septic tanks. The desludgeing tic tanks are probably the most widely applied period varies considerably from one to several treatment system in the world for treatment of years. Along with the low efficiency, the need for sewage from individual households. They can be periodical desludgeing is the main draw-back of pre-fabricated in fibre-glass or plastic, or con- the septic tanks. structed on-site in masonry or concrete. Nor- mally they are installed under the ground and 2.2. Anaerobic pond or lagoon the effluent is infiltrated into the soil, or discharged on the surface (gutters) or receiving In practice, the anaerobic pond is not used as a waters, eventually after some form of post-treat- treatment system alone. In conventional WSP ment, commonly the anaerobic filter. Septic systems, it receives the raw sewage and treatment tanks for single households usually have only is complemented with facultative and maturation one chamber. In Brazil the design of these units ponds, where more organic material as well as is regulated by a national norm (NBR 7229), pathogens (worm eggs and coliforms) are re- which states that the single chamber septic tanks moved. This configuration is amply applied in should have a volume such, that the reten- small and medium communities, especially in ) tion time is 24 h for flows up to 6000 L day 1, developing countries.