Sedimentary organic matter preservation: an assessment and speculative synthesis

Biological degradation of plastics: a comprehensive review.

Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

That major flows of energy occur along detrital pathways in all ecosystems is a recent recognition. In freshwater ecosystems, detritus or dead organic matter (217) has two possible sources: autochthonous detritus generated within the ecosystem and allochthonous detritus generated externally. This review is concerned with the breakdown of vascular plant detritus whether autochthonous, from aquatic vascular plants, or allochthonous, derived from riparian trees and herbs. The importance to the energetics of streams of vascular plant material from riparian vegetation was recognized in early studies by Nelson & Scott (184), Egglishaw (85), and Minshall (175). Organic matter budgets for various streams have provided quantitative data to support these early observations (96, 132, 182, 254). However, many low-order streams that lack canopies of riparian vegetation may be dominated by autochthonous primary production of nonvascular plant origin. (72, 176). Theoretical models (256) predict increasing importance of autochthonous production by periphyton and aquatic vascular plants for middle-order streams but less importance of these sources in very large streams, mainly due to light limitation. The relative dominance of allochthonous vs autochthonous sources has been shown to vary between stream systems and with local conditions within streams (72, 178). While vascular plant leaves have received most attention in stream research, there is growing recognition that wood is also important. The direct contribution of wood to stream energy budgets is minimal because wood is resistant to breakdown (e.g. 8, 251). However, woody debris is indirectly important because it creates habitat for aquatic organisms (5), promotes

Vascular plant breakdown in freshwater ecosystems

In seeking greater sustainability in water resources management, wastewater is now being considered more as a resource than as a waste-a resource for water, for plant nutrients, and for energy. Energy, the primary focus of this article, can be obtained from wastewater's organic as well as from its thermal content. Also, using wastewater's nitrogen and P nutrients for plant fertilization, rather than wasting them, helps offset the high energy cost of producing synthetic fertilizers. Microbial fuel cells offer potential for direct biological conversion of wastewater's organic materials into electricity, although significant improvements are needed for this process to be competitive with anaerobic biological conversion of wastewater organics into biogas, a renewable fuel used in electricity generation. Newer membrane processes coupled with complete anaerobic treatment of wastewater offer the potential for wastewater treatment to become a net generator of energy, rather than the large energy consumer that it is today.

Domestic Wastewater Treatment as a Net Energy Producer–Can This be Achieved?

A seven-year study addressed both the limitations on the rate and efficiency of biomass conversion, and the enrichment of biogas methane content. Energy crops examined included sorghum ( Sorghum bicolor ), napiergrass ( Pennisetum purpureum ), corn ( Zea mays ), and a sorghum/α-cellulose mix. High solids digestion work (25–30% effluent total solids (TS)) showed the need to address ammonia toxicity and trace nutrient limitations. Trace nutrient supplementation and control of the feedstock C/N ratio enabled stable operation of digesters at volatile solids (VS) loading rates up to 24 grams per kilogram reactor wet mass per day (gVS kg −1 day −1 ), with mean methane production rates of 7.5 L kg −1 day −1 . Acid-extractable metal concentrations were used as an indicator of bioavailable metals. Initial work with low solids digestion (8–10% TS) resulted in efficient VS conversion but low methane production rates. Subsequent work using trace nutrient supplementation enabled stable operation of intermittently-fed (three times per week) digesters at loading rates up to 12 gVS kg −1 day −1 , resulting in methane production rates up to 3.3 L kg −1 day −1 . Continuous feeding of corn at rates up to 18 gVS kg −1 day t-1 resulted in a mean methane production rate of 5.4 L kg−1 day-' with a 67% VS conversion efficiency. The maximum methane production rates for both the high solids and low solids systems are among the highest observed for biomass conversion. An in situ technique to enrich digester offgas was developed to take advantage of the differing solubilities of C0 2 and CH 4 , in which dissolved C0 2 was removed from the reactor in a recycled leachate stream and gas-stripped in an external stripper. Such a system easily enriched the remaining digester offgas to over 90% methane, and contents in excess of 98% were achieved. Quantitative evaluation of system variables defined the effects of leachate recycle rates, leachate alkalinity, and pH on the resulting offgas methane contents.

Methane fermentation of energy crops: maximum conversion kinetics and in situ biogas purification.

The major objective was to determine the rate and extent of algal degradation under simulated natural conditions. Decomposition of heterogeneous and unialgal cultures was studied under dark, anaerobic, constant-temperature laboratory conditions. Effects of high sulfate concentration, bacterial seedings, temperature, pH, and cell composition on the rate and extent of degradation were evaluated. After 200 days, decomposition of algal cultures was essentially complete, and the undecomposed particulate organic matter remaining was termed the refractory organic fraction. This fraction ranged from 20 to 60% of the ash-free dry weight for various culture with an average of 40%. The decomposition of the biodegradable organic fraction could be adequately described by first-order decay kinetics with a range for the decay constant k of 0.011-0.032/d with an average 0.022/day. The rate and extent of degradation were similar to those found by other investigators under aerobic decomposition conditions.

Anaerobic decomposition of algae

Biodegradability of modified plastic films in controlled biological environments

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.

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William J. Jewell

Papers

Methane fermentation of energy crops: maximum conversion kinetics and in situ biogas purification.

Anaerobic decomposition of algae

Biodegradability of modified plastic films in controlled biological environments

Methods for kinetic analysis of methane fermentation in high solids biomass digesters

Biotransformation of tetrachloroethylene by anaerobic attached-films at low temperatures