Filter Bed Material Research Articles

Comparative assessment of domestic wastewater treatment via Vermifiltration through various filter bed material.

Vermifilter (VF) is considered sustainable for rural areas; however, filter media is the most important but has been explored less. This study evaluated the performance of vermifilters in treating domestic wastewater (DWW) using various filter media, including areca nutshell (AS), rice straw (RS), dry leaves (DL), and chicken eggshells (ES). We compared the results with four different reactors: R1 (AS), R2 (RS), R3 (DL), and R4 (ES). DWW was applied with a hydraulic loading rate (HLR) of 1 m3/m2/d with Eisenia fetida earthworm species. The results showed the removal of biochemical oxygen demand (BOD) by 82%, 76%, 73%, and 87%; chemical oxygen demand (COD) by 75%, 73%, 72%, and 88%; phosphate by 36%, 25%, 27%, and 50%; sulfate by 56%, 54%, 53%, and 71% in R1, R2, R3, and R4, respectively. Simultaneously, R4 experienced a fivefold reduction in total bacteria and a sixfold reduction in total coliform. Moreover, the most exceptional filter media for vermifiltration is eggshells for the earthworm's growth and treatment efficacy.

ROOT ZONE TECHNOLOGY FOR WASTE WATER TREATMENT

In India, the primary cause of surface and ground water pollution is the discharge of untreated sewage, according to a 2007 study by the Central Pollution Control Board. In India, there is a significant delay between the generation and treatment of household wastewater. India's insufficient treatment capacity is not the only issue; there are also sewage treatment plants that exist, but they are not being maintained or operated. Because of poor design, inadequate maintenance, unreliable electricity supplies, absentee staff, and incompetent management, the majority of government-owned sewage treatment plants remain closed for the most part of the year. Usually, the wastewater produced in these places evaporates or percolates into the soil. Uncollected waste builds up in urban areas, releasing pollutants that seep into surface and groundwater and creating unsanitary conditions. Furthermore, industrialists prefer not to treat the waste produced and instead to dump it untreated into rivers in the areas where these treatment plants are located because the cost of treatment is so high.The Root Zone Treatment System (RZTS), sometimes referred to as the reed bed system or the constructed wetland system, is a sealed filter bed that is planted with vegetation that can grow in wetlands and is made up of a sand, gravel, and soil system, sometimes with a cohesive element. The wastewater flows through the filter bed, where biodegradation of the wastewater occurs, after the coarse and floating material has been removed. Complex physical, chemical, and biological processes that are the outcome of the interaction of wastewater, wetland plants, filter bed material, and microorganisms define the functional mechanisms in the soil matrix that are in charge of the mineralization of biodegradable matter.

OVERVIEW OF ROOT ZONE TECHNOLOGY

Central Pollution Control Board (2007) study found that discharge of untreated sewage is sthe single most important cause for surface and ground water pollution n India. There is a large gap between the generation and treatment of domestic wastewater in India. The problem is not only that India lacks sufficient treatment capacity but also that the existing sewage treatment plants are not operated and are not maintained. Most of the government-owned sewage treatment plants remain closed due to improper design, poor maintenance, or lack of reliable electricity supply to operate the plants, absentee employees, and poor management. The wastewater generated in these areas normally percolates in the soil or evaporates. The uncollected wastes accumulate in the urban areas cause unhygienic conditions and release pollutants that leach to surface and groundwater. Moreover, where these treatment plants exist, the cost of treatment is so high that industrialists favour not treating the waste generated and directly dumping the untreated waste into the rivers.The Root Zone Treatment system (RZTS) also known as the reed bed system or constructed wetland system is a sealed filter bed consisting of a sand/gravel/ soil system, occasionally with a cohesive element, planted with vegetation that can grow in wetlands. After removing coarse and floating material, the wastewater passes through the filter bed where biodegradation of the wastewater occurs. The functional mechanisms in the soil matrix responsible for the mineralization of biodegradable matter are characterized by complex physical, chemical and biological processes resulting from the combined effects of the filter bed material, wetland plants, micro-organisms and wastewater. The root zone treatment system also known as the reed bed or constructed wetland system is a sealed filter bed consisting of a sand /gravel/soil system.

The influence of moisture content on removal of H2S using the vermicompost biflter and analysis on microbial community

The utilization of biofilters provides a promising option, serving as an environment-friendly and economically beneficial strategy for waste gas abatement. In this study, the different moisture content of vermicompost with indigenous microorganisms as a filter bed material was evaluated for the performance of a biofilter in terms of hydrogen sulfide removal efficiencies and the bacterial dynamics. Maximum removal efficiency of hydrogen sulfide was conformed when the moisture content of the packing material was from 50% to 60%. By 16S rDNA gene-Illumina Miseq high-throughput sequencing technology, The Shannon and Simpon index of the vermicompost microbial community had significantly decreased after treating hydrogen sulfide. The predominant bacteria in vermicompost samples were Proteobacteria, Gemmatimonadetes, Bacteroidetes and Actinobacteria. Rhodanobacter and Mesorhizobium were isogenous with sulfur-oxidizing bacteria, which play a critical role in vermicompost biodegrading hydrogen sulfide.

From Renewable Biomass to Water Purification Systems: Oil Palm Empty Fruit Bunch as Bio-Adsorbent for Domestic Wastewater Remediation and Methylene Blue Removal

Oil palm empty fruit bunch fibers (OPEFBF), in three size ranges (small: 250–450 µm, medium: 450–600 µm, large: 600–800 µm), were investigated as a filter-bed material in biofilters for the removal of organic matter and nutrients. After saturation, these fibers (post) were used in the removal of methylene blue through batch processes. The batch adsorption tests included optimizing the adsorbent dosage (0.5–32.0 g/L) and contact time (2.5–60.0 min). Experimental data were fitted to various kinetic/isotherm models. Instrumental characterization of both raw and post fibers was conducted. Post fibers underwent morphological/compositional changes due to the presence of microorganisms and their byproducts. Efficiencies reached up to 94% for chemical oxygen demand (COD), 88.4% for total nitrogen and 77.2% for total phosphorus. In batch adsorption, methylene blue removal exceeded 90%, underscoring the effectiveness of small raw OPEFBF and large post OPEFBF. Kinetic models indicated that raw OPEFBF better fit the pseudo-first-order model, while post OPEFBF better fit the pseudo-second-order model. Both types of OPEFBF showed a good fit with the Freundlich model (higher R2, lower χ2 and SSE). Particularly, large post OPEFBF stood out as the most efficient adsorbent, achieving a maximum adsorption capacity of 12.02 mg/g for methylene blue. Therefore, raw/post OPEFBF could be an alternative to remove contaminants from wastewater.

Discard Coal as Filter Bed Material in Horizontal Subsurface Flow Constructed Wetlands: a Preliminary Study

Constructed wetlands (CWs) are engineered systems that use the natural functions of vegetation, substrate and microorganisms to treat wastewater. In coal mining regions, low calorific coals are dumped as discard. Left unattended, discard and slurry ponds contaminate surface and groundwater, cause erosion and sedimentation of particulates into nearby rivers and dams and contribute to atmospheric pollution and landslides. This study sought to investigate the use of South African bituminous discard as filter bed material for CW. A laboratory-scale horizontal subsurface flow (HSF) CW was supplied either nutrient-poor tap water (TW) or nutrient-rich advanced facultative pond (AFP) effluent, and quality of the treated water monitored over 6 months. Additionally, residual material from the discard coal filter bed was assayed after 6 months to establish substrate stability and to assess the contribution of phyto-biodegradation. Results showed successful establishment of P. australis on discard coal, better plant performance (measured as PSII quantum yield and biomass accumulation) and greater nutrient removal when fed AFP effluent. Discard coal filter bed material had greater ash content, sustained fixed carbon and C/N ratio with unchanged electrical conductivity (EC) and sulphate and phosphate concentration, indicative of balanced ion exchange. This, along with a > 70% reduction in NH4+-N concentration, yielded a final effluent within the general limit set by the South African authority for either irrigation or discharge, into a water resource that is not a listed water resource, for volumes up to 2000 m3 on any given day.

Study of effect of various temperatures on the abundance of ammonia oxidizing archaea and bacteria

Temperature plays significant role in the oxidation of ammonia in filtration units of recirculating aquaculture system. The impact of temperature on the abundance of ammonia oxidizing archaea and bacteria, and the expression of ammonia oxidizing gene (amoA) at specific temperature was evaluated. The broken earthen pot pieces used as filter bed materials of recirculating system, showing the presence of microorganisms were introduced in glass containers (5 pieces/5l) filled with synthetic wastewater and exposed to four different temperatures of 10, 20, 30 and 40°C for 40 days. The ammonia oxidation rate was minimum at 10°C. In 20, 30 and 40°C treatments, 99% ammonia was reduced on day-18, 8 and 18, respectively compared to the initial day. Fresh ammonium chloride (2 mM) was added twice to maintain the ammonia concentration in all treatments, except 10°C one. Nitrite-N level was < 1 mg/l at 10°C. The level was highest on day-22 at 20° and 40°C and on day-12 at 30°C. The nitrification was 10 days delayed at 20°C and 40°C compared to 30°C treatment. Concentration of nitrate-N was lowest at 10°C. Highest concentration of nitrate-N was observed on day-40 at 20°C and 40°C and day-26 at 30°C. Highest copy number of bacterial amoA was recorded at 30°C (2.59×107) followed by 20°C (4.08×106), 40°C (1.45×106) and 10°C (5.664×103). Archaeal amoA was highest at 30°C (7.47×103) followed by 40°C (2.98×102) and 20°C (46.8) treatments. Hence it may be concluded that 30°C temperature was optimum for the efficient and faster oxidation of ammonia in the present recirculating system.

Role of ionic concentration in the kinetic attachment of kaolinite and sand

Clay particle deposition in porous media is critical in many geotechnical applications that rely on filtration. The deposition of particles in the filter medium can result in clogging, which causes a reduction in the hydraulic conductivity of the filter medium. Particle attachment and detachment are a function of the interaction energy between clay particles and the filter bed material, which is frequently sand. These mechanisms are governed by the size of the clay particles as well as the solution chemistry. Batch kinetic adsorption tests were carried out to investigate the impact of ionic strength on the attachment of kaolinite to silica sand grains. Significant attachment of kaolinite was observed as ionic strength of the pore fluid increased and as the size of silica particles decreased. The short-term deposition of clay particles was not sensitive to ionic strength; however, in contrast, long-term deposition was a strong function of ionic strength.

Dynamic and initial head loss in full-scale wastewater filtration and measures to prevent long-term initial head loss

Abstract Dual media sand filters at Henriksdal WWTP began to show high head loss after five years of operation, especially during the fluidization for sorting the filter bed after backwashing. At that time, the filters were still clean and the dynamic head loss, due to clogging of the filters with suspended solids during a cycle of operation, had its lowest value. Dynamic head loss over the lower sand layer surface in the dual media sand filter was detected in some filter cycles of operation. The initial head loss in the filters has increased over the years. This type of head loss is constant during a cycle of operation and increases only slowly with time. It is due to different factors, as precipitation of ferric oxide hydroxide in the nozzle slots and on the grains in the filter bed, and accumulation of filter bed grains below the filter bottom. Different measures were tested to reduce or eliminate the initial head loss in the filters. Results from frequency diagrams showed that changing nozzles and removing of filter bed material from below the filter bottom were the most favourable actions. Backwashing more frequently and with more sequences also reduced the initial head loss.

The potential of granulated schwertmannite adsorbents to remove oxyanions (SeO32−, SeO42−, MoO42−, PO43−, Sb(OH)6−) from contaminated water

The mineral schwertmannite forms in acidic sulfate-rich environments (pH 2.9–4) and has a large sorption potential for oxyanions. In this study, we tested the properties of granulated schwertmannite produced from material obtained from a mine water treatment plant to remove SeO32−, SeO42−, MoO42−, PO43−, and Sb(OH)6−. We studied the kinetics of sulfate release from the granulated schwertmannite in suspensions containing various concentrations of HCO3− and its effect on the sorption kinetics of the oxyanions in synthetic solutions and industrial wastewater through batch experiment at acidic and neutral pH. We further tested the suitability of the granulated schwertmannite adsorbents as filter bed materials to remove SeO32−, MoO42−, and PO43− from synthetic solution via column experiments. Using a two-step pseudo-first order model, we were able to distinguish between an initial fast sorption process and a second slow reaction with characteristic reaction times (1/k) of ~10 min and ~10 h, respectively. Ligand exchange with surface bound sulfate is proposed to be responsible for the fast reaction while intraparticle diffusion and exchange with structural sulfate control the slow process. At neutral pH values, sulfate release is complete within one week, suggesting partial transformation of schwertmannite into a new ferric oxyhydroxide phase. The sorption kinetics were reflected in the results from the column experiments where only the fast sorbing fraction of surface sites participated in the retention of oxyanions at a mean residence times τ of 8 min with retention capacities of 149 μmol∙(g SHM)−1 for SeO32− and 242 μmol∙(g SHM)−1 for MoO42−. Hydraulic residence time increased the retention of PO43− with 213 μmol∙(g SHM)−1 at τ = 4.75 min and 508 μmol∙(g SHM)−1 at τ = 47.5 min. In summary, granulated schwertmannite turned out to be a promising adsorbent to remove the tested oxyanions from contaminated waters. Our results demonstrated that its application in filter bed systems needs to consider the trade-off between residence time and sorption kinetics. An increase in residence time clearly increases the retention capacity.

Novel coalescer design with bed of waste polymer fibers for liquid aerosol separation

This paper presents the development and optimization of a novel self-cleaning coalescer with drainage section, for liquid aerosol treatment. Four different waste polymer fibers were investigated as filter bed material for the novel coalescer: polyurethane (PU), polyethylene terephthalate of different origin (PET1 and PET2) and polypropylene (PP). Metal working fluid (MWF) mist from 6% of MWF emulsion was applied as liquid aerosol. Different bed thickness with and without preloading methods were tested to find an optimal separation efficiency for acceptable pressure drop. The applied air velocity through the coalescer was varied from 1 m/s to 5 m/s. Experimental results show that all investigated polymer materials could be successfully used as filter bed in the newly developed coalescer. The total separation efficiency is over 96% (between 96.73 and 99.98%) for almost all investigated conditions, except at the air velocity of 1 m/s, for PU and PP filter bed when it is 88.55% and 91.14%, respectively. Maximal separation efficiency for fraction 1 (0.2–0.3 µm) and 2 (0.3–1 µm) is 81.36% and 99.71%, respectively. The coalescer realizes maximal pressure drop of 4.75 kPa at 5 m/s air velocity in wet-steady state. The findings indicate that the novel coalescer could be easily optimized depending on inlet air quality and permissible level of PM1 in the outlet air.

The locally resolved filtration process for removal of phosphorus and micropollutants with GAC

Flocculation filtration in deep bed is one of the techniques to achieve advanced phosphorus removal. In addition to particle retention and phosphorus removal, granular activated carbon may serve as a filtration medium but also as an adsorption bed for micropollutants. The aim of the study was to investigate the simultaneous elimination of total phosphorus (TP) and selected micropollutants. In the first part of the study the target phosphorus threshold of 0.2 mg/L TP in the filtrate was achieved in a dual-bed media filter with anthracite and sand up to filter velocities of 16 m/h. By using a detailed sampling protocol and headloss curves, the influence of different operation modes on retention could be identified. Afterwards, the filter media were replaced by three types of GAC. In all filter beds substantial differences were found in the mode and efficiency of particle retention. This led to different filter run times, which is one of the main parameters for efficient and economical filter operation. Specific curves were generated for the different bed materials showing the dependency of filter run time on the specific load of suspended solids. Elimination of micropollutants in a GAC 4 × 8 mesh filter bed was low due to coarse grains and low empty bed contact time (EBCT). The application of GAC 8 × 14 mesh filter bed resulted in high elimination of most micropollutants. These results can be applied when choosing filter bed material. This may involve a compromise between filter run time and adsorption of micropollutants.

Diversity and Abundance of Ammonia-Oxidizing Bacteria and Archaea in a Freshwater Recirculating Aquaculture System

The role of ammonia-oxidizing bacteria and archaea was evaluated using amoA gene in a freshwater recirculating system. Broken earthen pot pieces (BEP) were used as filter bed material. Five archaeal and four bacterial operational taxonomic units were retrieved from amoA genes. Shannon-Weiner and Simpson indices were higher in archaeal amoA sequence compared with the bacteria. Subtype diversity ratio and subtype diversity variance were 0.522 and 0.008, respectively, for archaea and 0.403 and 0.015, respectively, for bacteria. In archaea, 50% amoA sequences showed 99%–100% similarity with the known sequences of ammonia monooxygenase subunit A of uncultured archaeon clones and thaumarchaeote. In bacteria, 84% sequences showed 99% similarity with amoA sequences of different uncultured bacterial clone and Nitrosomonadaceae. Absolute quantification showed that the abundance of archaea was 12-fold higher compared with bacteria. In this recirculating system, ammonia-oxidizing archaea and bacteria played a major role; BEP supported the growth of these ammonia-oxidizing microorganisms.

Fibrous deep-bed filtration for oil/water separation using sunflower pith as filter media

The application of agricultural by-products as filter bed material for the treatment of oily wastewater is preferable due to their biodegradability and availability. The goal of this paper was to examine the sorption and filtration properties of sunflower (Lat. Helianthus annuus) fibres in a raw state. SEM imaging and nitrogen adsorption at −196.15°C were applied to evaluate sorbent morphology. FTIR spectroscopy was used to determine the chemical composition of the pith. Studies on sorption in static conditions (pith fragmentation 1.5–4.0mm) allowed process kinetics to be specified. The experimental kinetics was fitted to pseudo first-order and pseudo second-order kinetics by a non-linear method. To better predict kinetic parameters, a new modified logistic equation was proposed. Sunflower fibre was found to have high oil sorption capacity (12.67g/g) and good retention properties.During filtration experiments, an artificial reservoir brine (0.1, 2.0, and 20.0g/l of oil) was used as an oil-in-water emulsion; sunflower pith was used as a filter medium. The influence of four process parameters (pH, bed height, initial oil concentration, and flow rate) on removal efficiency was studied. Oil concentrations were determined by gas chromatography. The results demonstrated that the prepared filters operated better at a lower pH, lower flow, a deeper bed material, and higher concentration of inlet oil. The filtration efficiency for the removal of crude oil from oily brines reached more than 99% in the initial stage of the process. The results obtained suggest that natural plant fibres can be an effective bed material to separate oil from oily brine.

Microbial removal of selected volatile organic compounds from the model landfill gas

Abstract Landfills of municipal waste are an important source of BTEXs in the atmosphere. Biodegrability of these compounds implies that biological methods, such as oxidation in landfill covers, may be an effective way to mitigate emission of these gases. The aim of the study was to evaluate the efficiency of BTEXs removal from landfill gas by biofiltration method and to analyze the influence of methane on BTEXs oxidation rate. The experiments were carried out at laboratory scale continuous flow system (microcosms) and in batch tests. A mixture of municipal waste compost and expanded clay pellets (1:1 of volume) was used as a filter bed material. The model landfill gas (50% vol. of CH4 and 50% vol. of CO2) purged through the microcosms was enriched with toluene (series 1) and all the BTEXs (series 2). The results of 7-month continuous flow experiment showed that removal efficiency of BTEXs in experimental columns ranged from 91 to 100% when the individual trace gases loading rates in model gas were in the range of 0.1-0.2 g m−2d−1. The rate of toluene removal, which followed the first order kinetics, depended on the presence of methane in treated gas. About 2-fold higher values of rate constant and 2.5-fold higher values of initial toluene removal rates were observed when no methane was present in the headspaces inside the vials used in the batch tests.

The effect of bed properties on methane removal in an aerated biofilter – Model studies

The capacity of laboratory-scale aerated biofilters to oxidize methane was investigated. Four types of organic and mineral–organic materials were flushed with a mixture of CH 4, CO 2 and air (1:1:8 by volume) during a six-month period. The filter bed materials were as follows: (1) municipal waste compost, (2) an organic horticultural substrate, (3) a composite of expanded perlite and compost amended with zeolite, and (4) the same mixture of perlite and compost amended with bentonite. Methanotrophic capacity during the six months of the experiment reached maximum values of between 889 and 1036 g m −2 d −1. Batch incubation tests were carried out in order to determine the influence of methane and oxygen concentrations, as well as the addition of sewage sludge, on methanotrophic activity. Michaelis constants K M for CH 4 and O 2 were 4.6–14.9%, and 0.7–12.3%, respectively. Maximum methanotrophic activities V max were between 1.3 and 11.6 cm 3 g −1 d −1. The activity significantly increased when sewage sludge was added. The main conclusion is that the type of filter bed material (differing significantly in organic matter content, water-holding capacity, or gas diffusion coefficient) was not an important factor in determining methanotrophic capacity when oxygen was supplied to the biofilter.

Earthworm cast as a promising filter bed material and its methanotrophic contribution to methane removal

The use of biocovers is a promising strategy toward mitigating CH 4 emission from smaller and/or older landfills. In this study, a filter bed material consisting of a mixture of earthworm cast and rice paddy soil in a biocover was evaluated. Although the CH 4 oxidation rate of the enriched paddy soil was 4.9 μg g-dry soil −1 h −1, it was enhanced to 25.1 μg g-dry soil −1 h −1 by adding an earthworm cast with a 3:7 ratio of earthworm cast:soil (wet weight). CO 2 was found as the final oxidation product of CH 4, and the mole ratio of CO 2 production to CH 4 consumption was 0.27. At a moisture content range of 15–40% and a temperature range of 20–40 °C, the CH 4 oxidation rates of the enriched mixture were more than 57% of the maximum rate obtained at 25% moisture content and 25 °C. By denaturing gradient gel electrophoresis analysis employing primers for the universal bacterial 16S rRNA gene, and terminal-restriction fragment length polymorphism analysis using primers for the pmoA gene, the bacterial and methanotrophic communities in the enriched mixture were mainly originate from paddy soil and earthworm cast, respectively. Both type I (mainly Methylocaldum) and type II methanotrophs (mainly Methylocystis) played important roles in CH 4 oxidation in the enriched mixture.

A novel procedure for the measurement of the kinetics of styrene biodegradation in a biofilter

AbstractA novel procedure for the study of the biodegradation kinetics of styrene is proposed in this paper. Microbial growth tests in the presence of styrene were performed using suspensions of microorganisms prepared from samples of filter bed material. This material had been directly withdrawn previously from an operating biofilter employed for the removal of styrene vapours from air. The kinetics tests were initiated immediately after transfer of filter bed samples from the active styrene filter bed to sealed flasks containing a mineral‐based medium, and were performed in such a manner that no enrichment technique or lag phase treatment was necessary to activate the desired biodegradation of styrene. The complete biodegradation of freshly introduced styrene was observed within a few hours of the filter bed's initial contact with the active microbial suspension. A novel theoretical development, as proposed in this study, has provided us with the means for estimating the yield coefficient of the biomass with respect to the styrene consumed and the kinetic coefficients of the growth rate without having to perform any analyses of the liquid phase after start‐up of the growth tests. Only monitoring of the styrene and carbon dioxide concentrations in the liquid and gas phases in the test flasks was necessary. The Haldane model, with inhibition expected at high styrene concentrations, was found to be the best to represent the biodegradation kinetics of styrene in this environment. Copyright © 2005 Society of Chemical Industry

Biofiltration of waste gases containing a mixture of formaldehyde and methanol

Several biofilters and biotrickling filters were used for the treatment of a mixture of formaldehyde and methanol; and their efficiencies were compared. Results obtained with three different inert filter bed materials (lava rock, perlite, activated carbon) suggested that the packing material had only little influence on the performance. The best results were obtained in a biotrickling filter packed with lava rock and fed a nutrient solution that was renewed weekly. A maximum formaldehyde elimination capacity of 180 g m(-3) h(-1) was reached, while the methanol elimination capacity rose occasionally to more than 600 g m(-3) h(-1). Formaldehyde degradation was affected by the inlet methanol concentration. Several combinations of load vs empty bed residence time (EBRTs of 71.9, 46.5, 30.0, 20.7 s) were studied, reaching a formaldehyde elimination capacity of 112 g m(-3) h(-1) with about 80% removal efficiency at the lowest EBRT (20.7 s).

Determination of the hydraulic residence time in a trickling biofilter filled with organic matter

Summary Biofiltration process using peat as media has been shown to be efficient for the treatment of agroindustrial, chemical, and municipal effluents. However, determining the hydraulic retention time of this process is difficult due to many factors. Generally tracer techniques are used, but they measure the molecular retention time instead of the hydraulic retention time; and depending on the case, the results can be wrong. In order to contribute to solving this problem, the objective of this research was to find a trustworthy and adequate technique to determine the hydraulic retention time for a process using a very adsorbent filter bed material (peat). An overview of the hydraulic behaviour of a peat bed filter related to their particular structural composition is presented. Then, based on these characteristics, two different techniques to determine the hydraulic retention time were developed and tested. The first, an indirect modified tracing technique and, the second, a direct one which determines the volume of liquid in treatment into the filter bed through the establishment of a relation between the volume of porosity and the volume of empty spaces (or not occupied by liquid) into the filter bed. The results obtained showed that the proposed techniques gave similar results and proved adequate