Brian K. Richards

MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL FIFTEEN YEARS AFTER SLUDGE APPLICATION

The increased use of sewage sludge as a fertilizer, combined with reports that large fractions of sludge-borne heavy metals cannot be accounted for several years after land application, indicates that more detailed study of potential mobility of these metals in soils is needed. A field site that

Effect of sludge-processing mode, soil texture and soil pH on metal mobility in undisturbed soil columns under accelerated loading.

The effect of sludge processing (digested dewatered, pelletized, alkaline-stabilized, composted, and incinerated), soil type and initial soil pH on trace metal mobility was examined using undisturbed soil columns. Soils tested were Hudson silt loam (Glossaquic Hapludalf) and Arkport fine sandy loam (Lamellic Hapludalf), at initial pH levels of 5 and 7. Sludges were applied during four accelerated cropping cycles (215 tons/ha cumulative application for dewatered sludge; equivalent rates for other sludges), followed by four post-application cycles. Also examined (with no sludge applications) were Hudson soil columns from a field site that received a heavy loading of sludge in 1978. Romaine (Lactuca sativa) and oats (Avena sativa) were planted in alternate cycles, with oats later replaced by red clover (Trifolium pratense). Soil columns were watered with synthetic acid rainwater, and percolates were analyzed for trace metals (ICP spectroscopy), electrical conductivity and pH. Percolate metal concentrations varied with sludge and soil treatments. Composted sludge and ash had the lowest overall metal mobilities. Dewatered and pelletized sludge had notable leaching of Ni, Cd and Zn in Arkport soils, especially at low pH. Alkaline-stabilized sludge had the widest range of percolate metals (relatively insensitive to soils) including Cu, Ni, B and Mo. Old site column percolate concentrations showed good agreement with previous field data. Little leaching of P was observed in all cases. Cumulative percolate metal losses for all treatments were low relative to total applied metals. Leachate and soil pH were substantially depressed in dewatered and pelletized sludge soil columns and increased for alkaline-stabilized and ash treatments.

Metal mobility at an old, heavily loaded sludge application site.

This study was undertaken to determine the present distribution and mobility of sludge-applied metals at an old land application site. Trace metals concentrations were determined for soils (using 4 M HNO3 extracts), soil leachates (collected with passive wick lysimeters over a 2.5-year period), and plant tissue from a field site which received a heavy loading of wastewater sludge in 1978 and an adjacent control plot. Blue dye was used to indicate preferential percolate flowpaths in the sludge plot soil for sampling and comparison with bulk soil metals concentrations. After nearly 20 years, metals in the sludge plot leachate were found at significantly greater concentrations than in the control plot, exceeding drinking water standards for Cd, Ni, Zn, and B. Annual metals fluxes were only a fraction of the current soil metal contents, and do not account for the apparent substantial past metals losses determined in a related study. Elevated Cd, Cu, and Ni levels were found in grass growing on the sludge plot. Despite heavy loadings, fine soil texture (silty clay loam) and evidence of past and ongoing metals leaching, examination of the bulk subsoil indicated no statistically significant increases in metals concentrations (even in a calcareous subsoil horizon with elevated pH) when comparing pooled sludge plot soil profiles with controls. Sampling of dyed preferential flow paths in the sludge plot detected only slight increases in several metals. Preferential flow and metal complexation with soluble organics apparently allow leaching without easily detectable readsorption in the subsoil. The lack of significant metal deposition in subsoil may not be reliable evidence for immobility of sludge-applied metals.

Long-term leaching of trace elements in a heavily sludge-amended silty clay loam soil

Analysis by ICP-MS of shallow groundwater collected at a field site in New York that had been heavily loaded with sewage sludge more than 15 years earlier revealed elevated concentrations of Cu, Zn, Sr, Rb, Mo, Cd, As, Cr, Ni, Sb, W, Ag, Hg, and Sn compared with a nearby control site. Enhanced leach

Losses of manurial nitrogen in free-stall barns

Abstract The objectives of this study were to measure nitrogen losses from dairy manure in free-stall dairy barns which were scraped once a day and to compare these results with a previously published model predicting nitrogen losses in the barn. From two farms floor samples were collected once a week from manure just scraped from each alley. The results of this sampling programme indicated that nitrogen losses were minimal when the average daily air temperature in the barn was below 5°C. Between 5°C and 25°C the losses increased with temperature, reaching 40% to 60% of the total nitrogen in the manure. The measured losses were similar to the models predictions below 10°C but were greater than the predicted values at higher temperatures. Since the model considered only urea and ammonia nitrogen as available for loss, it is apparent from the farm data that under warm conditions significant quantities of non-urea organic nitrogen in both the urine and faeces are ammonified within 24 h.

Trace metal accumulation by red clover grown on sewage sludge-amended soils and correlation to Mehlich 3 and calcium chloride-extractable metals

With the availability of sensitive multielement analytical capability, it is no longer essential to use chemically aggressive soil tests to extract measurable levels of most trace elements. However, the relative abilities of mild and aggressive extractants to assess metal bioavailability in soils have rarely been compared. A greenhouse experiment was carried out to compare a mild soil extractant (hot 0.01 M CaCl2) with an aggressive one (Mehlich 3) for predicting accumulation of trace metals by red clover (Trifolium pratense L.). Clover was grown on large columns of nonacid fine-textured and acid coarse-textured soils that had been amended several years earlier by a heavy application of sewage sludge products, and pH was subsequently adjusted using CaCO3 or H2SO4. The soil extractants (CaCl2 and Mehlich 3) and clover tissue were analyzed for trace metals (As, Cd, Mo, Cu, Ni, Mn, Pb, and Zn) by axial-view ICP spectrophotometry. Linear regression analyses were performed to relate the concentration of each trace element in the red clover tissue to the concentration extracted from the soil. The results indicate that CaCl2 extraction is more reliable than Mehlich 3 extraction when evaluating plant availability of trace elements in soils with a wide range of properties (especially pH). The strongly acidic nature of the Mehlich 3 extractant caused large quantities of metals such as Zn, Cd, Cu, and Ni to be extracted from metal-contaminated soils even when the plant availability of these metals was low because of near-neutral soil pH or high clay and organic matter content. Conversely, in coarse-textured and acid soils containing lower total concentrations of metals, plant-available metals were often relatively high, yet Mehlich 3 frequently extracted smaller quantities of metals from these soils than from the near-neutral soils. We conclude that dilute CaCl2 is preferable to Mehlich 3 as a universal soil extractant for estimating short-term trace metal availability to crops.

METHODS FOR KINETIC ANALYSIS OF METHANE FERMENTATION IN HIGH SOLIDS BIOMASS DIGESTERS

Methods are presented for kinetic analysis of anaerobic biomass reactors. In some cases, assumptions implicit in kinetic analysis techniques developed for conventional dilute digestion modes are not applicable to systems operating at high rates and/or high solids concentrations. As a result, modified definitions are presented for CST digester retention times and first order kinetic coefficients. Procedures are presented for converting biogas data to standard conditions. Two novel methods for quantifying mass removals, based on direct measurement of reactor mass losses and on biogas production, allow rapid determination of mass removal rates and detection of gas leakage. The use of a per unit mass basis for reporting concentrations and kinetics is recommended.

High solids anaerobic methane fermentation of sorghum and cellulose

Abstract Sorghum and sorghum/cellulose mixtures were digested at 55°C at effluent solids contents over 25% total solids (TS). Sorghum alone as a feedstock led to excess NH 3 accumulation, while a sorghum/alpha-cellulose mix controlled NH 3 at acceptable levels. Trace nutrient supplementation was required for stable digestion. Stable performance at organic loading rates (OLR) of 18 and 24 gVS kg −1 day −1 resulted in steady methane production rates of 5.7 and 7.51 CH 4 kg −1 day −1 , respectively. The efficiency of volatile solids (VS) conversion to biogas was 74.8% and 72.1% at organic loading rates (OLR) of 18 and 24 gVS kg −1 day −1 , respectively.

Quantifying colloid retention in partially saturated porous media

[1] The transport of colloid-contaminant complexes and colloid-sized pathogens through soil to groundwater is of concern. Visualization and quantification of pore-scale colloid behavior will enable better description and simulation of retention mechanisms at individual surfaces, in contrast to breakthrough curves which only provide an integrated signal. We tested two procedures for quantifying colloid movement and retention as observed in pore-scale image sequences. After initial testing with static images, three series of images of synthetic microbead suspensions passing through unsaturated sand were examined. The region procedure (implemented in ImageJ) and the Boolean procedure (implemented in KS400) yielded nearly identical results for initial test images and for total colloid-covered areas in three image series. Because of electronic noise resulting in pixel-level brightness fluctuations the Boolean procedure tended to underestimate attached colloid counts and conversely overestimate mobile colloid counts. The region procedure had a smaller overestimation error of attached colloids. Reliable quantification of colloid retention at pore scale can be used to improve current understanding on the transport mechanisms of colloids in unsaturated porous media. For example, attachment counts at individual air/water meniscus/solid interface were well described by Langmuir isotherms.

Reporting on Marginal Lands for Bioenergy Feedstock Production: a Modest Proposal

Growing bioenergy feedstocks can provide a long-term sustainable production system for marginal land resources and is essential for minimizing food vs. fuel competition for prime croplands. However, the term “marginal” is too often used in research reports without being defined. We here suggest that clearly specifying the biophysical factors and agroeconomic context contributing to marginality will greatly enhance the utility and comparability of published research.