I. Ruiz

MUNICIPAL WASTEWATER TREATMENT IN AN ANAEROBIC DIGESTER‐CONSTRUCTED WETLAND SYSTEM

Abstract An experimental plant was constituted for an upflow anaerobic sludge blanket (UASB) reactor with an active volume of 25.5 m3 and two horizontal constructed wetlands of 75 m2 each. The first wetland was a superficial flow (SF) system and the second a subsurface flow (SSF) system. The UASB reactor was fed with 60–73 m3 d−1 of raw domestic wastewater from the municipal treatment plant (MTP) of the city of Santiago de Compostela. Part of the effluent from the UASB reactor (15 m3 d−1) passed through the SF wetland and then went into the SSF. In the first ten months of operation, from July 2005 to April 2006, the hydraulic retention time for the UASB system was in the range of 7–14 hours and the organic loading rate (OLR) was between 0.3 and 1.1 g COD l−1 d−1. The organic load in the wetlands was in the range of 5–40 g BOD5 m−2 d−1. The global results achieved for total suspended solids (TSS), chemical oxygen demand (COD) and biological oxygen demand (BOD5) removal efficiencies were 85–96%, 65–90% and 69–93%, respectively. The average monthly concentrations in the final effluent were 8–73 mg BOD5 l−1, 25–120 mg COD l−1 and 6–20 mg TSS l−1. The global balance of VSS in the UASB reactor shows that purge accounted for 19.6% and hydrolysis accounted for 47.7% of influent VSS. Data from suspended solids balance in the constructed wetlands, at the end of the operation period, show a TSS and VSS accumulation of 48% and 16%, respectively.

Methanogenic toxicity in anaerobic digesters treating municipal wastewater

In upflow anaerobic sludge blanket (UASB) digesters treating raw sewage at low temperatures, the sludge progressively lost methanogenic activity, indicating the possibility of methanogenic activity inhibition caused by wastewater constituents. To check this fact, batch and semi-continuous methanogenic toxicity assays were carried out with raw and centrifuged sewage. Permanent methanogenic toxicity on anaerobic sludge of approximately 50% was found when the sludge exposure to wastewater was renewed in a semi-continuous way. A stronger methanogenic inhibition of about 70-100% was observed when an active anaerobic sludge was exposed to mixed liquor from the UASB digester treating municipal wastewater. Suspended solids removal from sewage slightly reduced methanogenic toxicity. Effective concentration of municipal wastewater that caused a 50% reduction in methanogenic activity was estimated to be in the range of 150-200 mg CODl(-1). As methanogenic inhibition appeared to be related to remaining COD, higher methanogenic toxicity in digesters operating with low conversion efficiency will be expected.

Heavy metal removal in an UASB-CW system treating municipal wastewater.

The objective of the present study was to investigate for the first time the long-term removal of heavy metals (HMs) in a combined UASB-CW system treating municipal wastewater. The research was carried out in a field pilot plant constituted for an up-flow anaerobic sludge bed (UASB) digester as a pretreatment, followed by a surface flow constructed wetland (CW) and finally by a subsurface flow CW. While the UASB showed (pseudo) steady state operational conditions and generated a periodical purge of sludge, CWs were characterised by the progressive accumulation and mineralisation of retained solids. This paper analyses the evolution of HM removal from the water stream over time (over a period of 4.7 year of operation) and the accumulation of HMs in UASB sludge and CW sediments at two horizons of 2.7 and 4.0 year of operation. High removal efficiencies were found for some metals in the following order: Sn > Cr > Cu > Pb > Zn > Fe (63-94%). Medium removal efficiencies were registered for Ni (49%), Hg (42%), and Ag (40%), and finally Mn and As showed negative percentage removals. Removal efficiencies of total HMs were higher in UASB and SF units and lower in the last SSF unit.

Characteristics and anaerobic treatability of municipal and industrial estate wastewaters

Centralised treatment of domestic sewage coming from A Coruña city (250000 inhabitants) and several medium and small size villages (up to 15000 inhabitants) together with industrial estate effluents was planned. The area population is about 380000 inhabitants and the overall effluent will reach about 600000 equivalent inhabitants. The characteristics of different streams will determine the treatment alternatives and pre-treatment requirements for individual streams. Domestic sewage from A Coruña showed medium to high organic content (741 mg l-1 of COD), while domestic sewage from small villages (Lorbé, Oleiros) and industrial estates (Sabón, A Grela) showed a high variability in organic content, with COD concentration ranging from 100 to 4000 mg l-1 or more. About 50-65% COD corresponded to suspended solids, although some industrial effluents showed lower values. Characterized industrial effluents had lower concentration of nutrients (nitrogen and phosphorus) and higher concentration of sulphate and alkalinity than domestic sewage. Anaerobic biodegradability was about 80% for domestic sewage, and ranged from 20 to 100% for industrial estate effluents. Furthermore, some industrial streams showed high microbial toxicity. The results indicate that anaerobic treatment of these effluents is of interest. However, specific pre-treatment for some individual streams or the adoption of minimisation and prevention measures in order to reach total or partial detoxification is suggested.

Integrating pretreatment and denitrification in constructed wetland systems

The aim of this work was to study the operational characteristics and the efficiency of a compact constructed wetland system for municipal wastewater treatment that integrates denitrification in the pre-treatment unit. The proposed system was simulated by two units in series with effluent recirculation, the first one being an anoxic digester, conceived as a hydrolytic up flow sludge bed for solids hydrolysis and denitrification, and the second one a sand column that simulated the operation of a vertical flow constructed wetland. The hybrid system consisted of two small columns of 4 and 10.2cm in diameter (anoxic digester and vertical flow unit, respectively). The unplanted system was operated successively with synthetic and real municipal wastewater over a period of 136days. Hydraulic loading rate ranged from 212 to 318mm/day and surface loading rate from 122 to 145g/m2·day of chemical oxygen demand and 10-15g/m2·day of total nitrogen for the overall system. The overall system reached removals of 91% to 99% for total suspended solids, chemical oxygen demand and biochemical oxygen demand whilst total nitrogen removal ranged from 43% to 61%. In addition to suspended solids removal (up to 78%), the anoxic digester provided high denitrification rates (3-12gN/m2·day) whilst the vertical flow unit provided high nitrification rates (8-15gN/m2·day). Organic matter was mainly removed in the anoxic digester (63-82% chemical oxygen demand) and used for denitrification. Final effluent concentration was lower for ammonia (7.4±2.4mgN/L on average) than for nitrate (19.8±4.4mgN/L), denitrification appearing as the limiting step in nitrogen removal in the system. CH4 or N2O emissions were not detected in any of the units of the system indicating very low greenhouse gas emissions.