P. Griffin

Complementary methods to investigate the development of clogging within a horizontal sub-surface flow tertiary treatment wetland.

A combination of experimental methods was applied at a clogged, horizontal subsurface flow (HSSF) municipal wastewater tertiary treatment wetland (TW) in the UK, to quantify the extent of surface and subsurface clogging which had resulted in undesirable surface flow. The three dimensional hydraulic conductivity profile was determined, using a purpose made device which recreates the constant head permeameter test in-situ. The hydrodynamic pathways were investigated by performing dye tracing tests with Rhodamine WT and a novel multi-channel, data-logging, flow through Fluorimeter which allows synchronous measurements to be taken from a matrix of sampling points. Hydraulic conductivity varied in all planes, with the lowest measurement of 0.1md(-1) corresponding to the surface layer at the inlet, and the maximum measurement of 1550md(-1) located at a 0.4m depth at the outlet. According to dye tracing results, the region where the overland flow ceased received five times the average flow, which then vertically short-circuited below the rhizosphere. The tracer break-through curve obtained from the outlet showed that this preferential flow-path accounted for approximately 80% of the flow overall and arrived 8h before a distinctly separate secondary flow-path. The overall volumetric efficiency of the clogged system was 71% and the hydrology was simulated using a dual-path, dead-zone storage model. It is concluded that uneven inlet distribution, continuous surface loading and high rhizosphere resistance is responsible for the clog formation observed in this system. The average inlet hydraulic conductivity was 2md(-1), suggesting that current European design guidelines, which predict that the system will reach an equilibrium hydraulic conductivity of 86md(-1), do not adequately describe the hydrology of mature systems.

ANAEROBIC TREATMENT OF DOMESTIC WASTEWATER IN TEMPERATE CLIMATES: TREATMENT PLANT MODELLING WITH ECONOMIC CONSIDERATIONS

Although research suggests that anaerobic treatment of low-strength domestic wastewater is possible in temperate climates, to date, full-scale applications have only been pioneered in hot regions. However, burgeoning environmental legislation in developed countries is giving the impetus to develop anaerobic wastewater treatment systems due to potential economic and environmental benefits they hold over traditional aerobic techniques. In this paper a design rationale for low-temperature, low-strength (COD < 1,000 mgl(-1)), two-phase anaerobic wastewater treatment is developed through empirical modelling of data from published research, and from assumptions arising from a literature review. Model calculations are applied to typical domestic sewage characteristics at two different flow rates, based on population equivalents. Results indicate that soluble COD production in the model hydrolytic tank are similar to those achieved in pilot scale plants in the Netherlands. Model anaerobic reactor sludge characteristics are similar to those achieved in pilot and full-scale anaerobic reactors treating low-strength wastewaters. Indicative cost figures for a two-phase anaerobic treatment plant are given, but are incomplete without an assessment of the cost of post-treatment processes. Anaerobic treatment is likely to become more attractive in the future as new legislation relating to sludge disposal and renewable energy generation are introduced.

Model Study of Short-Term Dynamics of Secondary Treatment Reed Beds at Saxby (Leicestershire, UK)

Abstract Relatively simple black-box models, such as the well-known k-C* model, are commonly applied to design horizontal sub-surface flow constructed treatment wetlands. Important shortcomings of this model are the oversimplification of reality on the one hand, and the inability to predict short-term effluent dynamics on the other. A possible solution for these drawbacks could be the application of dynamic compartmental models. This article reports on the calibration requirements and the simulation results of such a dynamic model. A quantitative sensitivity analysis was used to identify the most sensitive parameters after which model predictions were optimized by adjusting those parameter values. Model fits were acceptable but missed some of the short-term dynamics observed in reality. At this point, it might therefore still be unwise to use the model as a design tool. Further model adjustments and calibration efforts are needed to enhance its reliability.

REED BED TREATMENT FOR SEWER DYKES

Abstract Prior to 1987, the domestic waste from 15 dwellings and farmyard waste from the Leicestershire village of Little Stretton discharged via a septic tank into a sewer dyke. The discharge from the sewer dyke had a deleterious affect on the River Sence downstream. In summer 1987, the sewer dyke was replaced by a reed bed system comprising eight gravel terraces in series, each approximately 12.5m long and 2.0m wide. The reed bed was established using pot-grown seedlings with almost 100% success and excellent growth has been maintained since. Despite problems with greater than anticipated loads of farm waste, the reed bed has averaged better than 60% removal of BOD and solids, and as a consequence downstream river quality has improved considerably. Constructional and operational problems of this type of reed bed are identified and flows, loadings and performance detailed.

Long term monitoring of constructed wetlands using an NMR sensor

Subsurface flow wetlands have become a popular technology for the treatment of waste water all over the world. These systems become clogged over time, and must be renovated at great expense. We present a nuclear magnetic resonance sensor which is sufficiently small and inexpensive that several of them could be embedded in a constructed wetland to allow spatially resolved long term continual monitoring of the clogging process. We demonstrate the suitability of this sensor by first measuring NMR of sludge from an operational wetland, and secondly by monitoring the evolution of the fluids NMR spin lattice relaxation time (T1) during clogging in a model wetland. Measurement of clogging rates in two locations are made and found to be 10.7×10−2min−1 and 4.9×10−3min−1 for regions near the inlet and the centre respectively.