Anaerobic digesters as a pretreatment for constructed wetlands

doi:10.1016/j.ecoleng.2008.02.001

Ecological Engineering

Volume 33, Issue 1, 1 May 2008, Pages 54-67

https://doi.org/10.1016/j.ecoleng.2008.02.001 Get rights and content

Abstract

The most commonly used pretreatment technologies for constructed wetland (CW) treatment of domestic sewage are septic tanks (ST) and Imhoff tanks (IT). These technologies have frequently suffered from failures and even in normal operation they offer insufficient removal of solids. As a result, combined ST-CW or IT-CW can experience substrate clogging, especially when high organic loads are applied. In the last 7 years, the operation of combined systems using high-rate anaerobic digesters as a pretreatment and CW as a post-treatment has been reported. A review of the literature indicates that CW in these combined systems operates with a similar organic loading rate (on a chemical oxygen demand basis) but with a lower total suspended solid (TSS) loading rate. In these combined systems, the TSS loading rate is 30–50% less than that applied in CW combined with classical pretreatment technologies. A low TSS loading rate could prevent substrate clogging in CW.

This work presents the results of different case studies on the treatment of municipal wastewater with high-rate anaerobic systems. Our interest is focused on the capacity of these systems for removing suspended solids, and therefore on their potential as an appropriate pretreatment to avoid clogging in constructed wetlands and to improve efficiency. Average and 95 percentile TSS concentrations of anaerobic treated wastewater were below 60 and 100 mg/l, respectively, for all configurations. Therefore, the use of high rate anaerobic systems as a pretreatment for constructed wetlands could delay gravel bed clogging. Furthermore, according to the level of organic matter removal, anaerobic pretreatment provided a 30–60% reduction in the required wetland area. Both treatment alternatives can be combined to develop low-cost, robust, and long-term systems for treating municipal wastewater.

Introduction

Sustainability of sanitation systems should be related to low cost and low energy consumption and, in some situations, low mechanical technology requirements. Decentralised and low-cost processes are considered to be a better choice for rural areas (Lens et al., 2001). Anaerobic digesters and constructed wetlands are treatment systems with a very small energy input, low operational cost, and low surplus sludge generation (Sperling, 1996, Kadlec et al., 2000, Lens et al., 2001, Hoffmann et al., 2002). These characteristics, together with low technological requirements, make them particularly suitable for decentralised wastewater treatment in rural areas.

The costs of construction, installation, and operation of anaerobic digesters are lower than those of conventional aerobic units because anaerobic digesters do not require expensive equipment for process maintenance and control. In fact, if the environmental conditions inside the digester are adequate, anaerobic processes are mainly self-controlled. Additionally, the production of excess sludge is minimal, and energy balances are quite favourable, even when heating is required, due to the production of methane (Foresti, 2002).

The disadvantage of anaerobic digesters is that additional treatment is necessary to polish and lower the pollution load. Even in tropical regions (Sousa et al., 2001), and mainly in cool to temperate climate regions (Álvarez et al., 2003), the effluent of UASB (up flow anaerobic sludge blanket) systems requires an effluent post-treatment to reduce organic mater, nutrients, and pathogenic microorganisms. In the case of operating temperatures below 20 °C, UASB systems are good at removing suspended solids; however, acetic acid accumulation in the effluent reduces the COD (chemical oxygen demand) and BOD (biological oxygen demand) removal efficiencies (Álvarez, 2004, Álvarez et al., 2006).

It is of great interest to combine wetland systems with anaerobic digesters in order to obtain sufficient treatment efficiency. The most commonly used anaerobic technology for municipal wastewater treatment is the UASB (Lettinga, 2001, Foresti et al., 2006, Van Haandel et al., 2006). There are several studies of systems combining anaerobic pretreatment and constructed wetlands, which are assessed in Section 4. UASB reactors are the referent pretreatment anaerobic technology used in these combined systems. However, other anaerobic technologies may be used as sewage pretreatment for constructed wetlands. The hydrolytic upflow sludge bed reactor (HUSB) is a promising alternative.

However, constructed wetlands (CW) are land-intensive treatment systems. The use of an appropriate anaerobic pretreatment before constructed wetland treatment can reduce the construction cost by about 36–40%, due to the fact that anaerobic treatment reduces the influent organic matter and therefore the area required for CW is decreased (Barros and Soto, 2002). Both anaerobic and wetland treatment approaches are characterized by low construction and operation costs, low excess sludge, and low energy demand. Therefore, both treatment technologies are complementary and highly sustainable. Limited organic removal efficiency in anaerobic digesters is compensated by high efficiency in CW, while anaerobic digesters present minimal area requirements, generally less than 0.1 m²/p.e. for UASB (Kivaisi, 2001).

Studies have shown that one of the most important operational handicaps of constructed wetlands is gravel bed clogging; this may occur after several years, resulting from the treatment of raw or poorly pretreated urban wastewater. Suspended solids that are not removed in a pretreatment system are effectively removed by filtration and settlement within the first few metres beyond the inlet zone. Thus, a high level of total suspended solid (TSS) removal in anaerobic pretreatment would contribute to avoiding or reducing wetland clogging problems, reinforcing constructed wetland sustainability (Vymazal, 2005, Caselles-Osorio et al., 2007).

The aim of this work is to analyse and discuss the potential of high-rate anaerobic digesters as a pretreatment for municipal wastewater that will later be treated in constructed wetlands. First, a brief analysis of clogging phenomena in CW is presented, and the pretreatment technologies most often used in combination with CW are discussed, focusing on their potential for reducing the quantity of suspended solids introduced into constructed wetlands. Next, the authors review the literature on systems combining anaerobic digesters and CW. Finally, detailed case studies on anaerobic pretreatment of municipal wastewater are presented, focusing on the efficiency of suspended solid removal and the potential of anaerobic digesters for preventing clogging and reducing CW area.

Section snippets

Substrate clogging in constructed wetlands

Substrate clogging encompasses several processes that lead to a reduction of the infiltration capacity of the gravel bed after several years of operation. In horizontal flow (HF) wetlands, apparent clogging and subsequent ponding near the inlet of the treatment cells dampen the remarkable performance of the system. This may occur after few years of operation (Dahab and Surampalli, 2001, Caselles-Osorio et al., 2007). In vertical flow (VF) wetlands, clogging of the substrate matrix critically

Pretreatment alternatives for constructed wetlands

The main objective of pretreatment or primary treatment is the reduction of suspended solids in wastewater, although additional treatment effects leading to organic content reduction and, in some cases, the hydrolysis and stabilization of the generated sludge are obtained. In this way, some pretreatment technologies can reach up to 50% COD or BOD removal. Furthermore, from a general point of view, pretreament operations are considered to be a convenient means of ensuring the correct operation

CW post-treatment of anaerobically treated sewage

Table 2 shows the main design and operating characteristics of various constructed wetlands for UASB-CW combined systems found in the literature. A dozen UASB-CW applications were described, although there is only information about influent TSS for a few systems, as can be seen by comparing Table 2, Table 1. In addition, the operational period reported in these studies is not long enough (the maximum operation period was 3 years) to conclude whether anaerobic pretreatment can prevent gravel bed

Anaerobic digestion processes and up flow anaerobic digesters

The UASB reactor is the most commonly used anaerobic technology for domestic sewage treatment; and the hydrolytic upflow sludge bed (HUSB) is an option to be considered. These digesters have similar design features, but are primarily differentiated by their operational conditions. Both UASB and HUSB can be operated as a single unit or as a combined two-step or hybrid system (see Fig. 1).

In upflow mode reactors like the HUSB and UASB, raw or pretreated wastewater enters the bottom of the

Influence of anaerobic pretreatment on constructed wetland area

Anaerobic pretreatment has two important consequences for the quality of influent wastewater in a constructed wetland. The first one is the high TSS removal and the maintenance of TSS concentration in the pretreated wastewater so that it is below 100 mg/l, as indicated above.

A second consequence is the decrease in the influent COD concentration to the wetland by an amount that varied from 30 to 90%, depending on the type of anaerobic digester used, wastewater characteristics, and operational

Conclusions

One of the most significant handicaps of constructed wetlands for urban wastewater treatment is gravel bed clogging after a few years of operation with poor waste pretreatment or high organic loading rates. Another disadvantage of constructed wetlands is that a large superficial area is required. Both handicaps can be minimised with an appropriate anaerobic pretreatment.

Anaerobic plants may be operated either as hydrolytic or methanogenic digesters. Hydrolytic digesters, at an HRT of 3–5 h,

Acknowledgments

This work was supported by project CTM2005-06457-C05-02/TECNO from the Ministery of Education and Science of Spain.

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Anaerobic digesters as a pretreatment for constructed wetlands

Abstract

Introduction

Section snippets

Substrate clogging in constructed wetlands

Pretreatment alternatives for constructed wetlands

CW post-treatment of anaerobically treated sewage

Anaerobic digestion processes and up flow anaerobic digesters

Influence of anaerobic pretreatment on constructed wetland area

Conclusions

Acknowledgments

Bioresour. Technol.

Water Sci. Technol.

Water Sci. Technol.

Ecol. Eng.

Water Res.

Environ. Pollut.

Water Res.

Desalination

Ecol. Eng.

Ecol. Eng.

Water Res.

Ecol. Eng.

Ecol. Eng.

Bioresour. Technol.

Water Sci. Technol.

Ecol. Eng.

Water Res.

Ecol. Eng.

Ecol. Eng.

Water Sci. Technol.

Water Res.

Anaerobic hydrolysis of a municipal wastewater in a pilot-scale digester

Water Sci. Technol.

Performance of an UASB-Digester system treating domestic wastewater

Environ. Technol.

Anaerobic systems for domestic wastewater treatment in rural areas

Depuración natural avanzada en núcleos rurais

Performance of an anaerobic digester-wetland system for a small community

Influence of surface layer on hydrology and biology of gravel bed vertical flow constructed wetlands

Water Sci. Technol.

Performance of two macrophyte species in experimental wetlands receiving variable loads of anaerobically treated municipal wastewater

Water Sci. Technol.