A cost-effective alternative to lagoon dredging.
Enzyme-Tomahawks are composed of enzymes, billions of microorganisms and micro-nutrients to accelerate the biodegradation of the sludge layer.
Typically, enzymes are supplied as an enzyme solution, which is injected into the recycle flow. However, in the case of a lagoon, the flow velocity within the lagoon will result in enzyme washout, preventing it from acting on the sludge layer at the bottom of the lagoon.
The schematic above shows the Enzyme-Tomahawks being deployed within the sludge layer of a typical lagoon.
In order to provide enough retention time of the enzyme mixture with the sludge layer, the mixture of enzymes, microorganisms and micro-nutrients will be supplied as solid Enzyme-Tomahawks, that will bury themselves into the sludge layer, when added from the top of the water layer, and release the enzymes, microorganisms and micro-nutrients within the sludge layer, while the nozzles in the water column will inject micro-bubbles of air and water into the water column. These micro-bubbles will provide much-needed dissolved oxygen and in conjunction with the enzyme release, within the sludge layer by the Enzyme-Tomahawks, the sludge layer will begin to biodegrade aerobically.
The schematic above shows the Enzyme-Tomahawks combined with the aeration system dissolving the sludge layer.
Typically, sludge depths less than 2 ft do not require dredging, since they do not contribute to either BOD or ammonia levels.
A large pulp and paper mill in the Southeastern US had significant biosolids accumulation in the quiescent zone of their Aerated Stabilization Basin that hindered treatment plant efficiency and performance. The wastewater system consisted of a primary clarification system with sludge removal via belt press located near the mill site that captures fiber and TSS from the combined paper and pulp mill effluent streams and a secondary biological treatment system located approximately 2.4 km west of the facility.
The primary clarifier effluent was pumped to an inlet box at the secondary treatment system and fed into the first run of the aerated stabilization basin. The 165,921 m2 Aerated Stabilization Basin had three aeration zones, separated by curtains that caused the flow to serpentine through the system. 32 surface mounted aerators with 2,400 total connected horsepower supplied dissolved oxygen to the biological treatment system. After aerated treatment, wastewater flows through a quiescent zone 762 meters long and 91 meters wide (69,342 m2) . This quiescent zone was designed to allow sludge to accumulate in the basin while treated effluent passed to the final outfall.
Enzyme Tomahawks were placed within the sludge layer of the quiescent zone area, two feet apart from each other, in areas with significant sludge depth. Sixteen buoys were installed in the quiescent zone area (762 m long and 91 m wide), and measurements of clear water depth were made between April and July.
The figure below shows the change in sludge depth (in mm), over a four month time period, for the 16 locations within the quiescent zone area.
The average sludge depth reduction was 247 mm, which resulted in almost 17,000 m3 of sludge destruction over the four month time period. This saved substantial dredging cost for the mill in addition to costs associated with dewatering and landfill disposal of the sludge.
Odor emission are mainly from hydrogen sulfide,
although other sulfur compounds such as mercaptans and disulfides may also exist.
Odor Control in Wastewater
Sulfur occurs may occur in several oxidative states, these include, Hydrogen Sulfide, elemental sulfur and sulfate. Sulfate is typically the form of sulfur found in the water reservoirs. Sulfate is the most reduced state of sulfur and the primary reason for sulfate is that bacteria have oxidized the other forms of sulfur to the sulfate state. Sulfate has no odor and is highly soluble in water in various sulfate compounds.
Sulfur reducing bacteria can reduce sulfate converting it into hydrogen sulfide in an environment devoid of oxygen. Since the deeper layers of water in the reservoir contains no dissolved oxygen, when a carbon source is introduced to a water containing sulfate, sulfide conversion can be produced by biological activity. The higher the dissolved organic concentration in the reservoir, the more prevalent will be the organic and biological sources necessary for this conversion. The chemical equation for sulfide conversion from sulfate is shown below:
SO4-2 + 2C organic + 2H20 + microbial activity → H2S + 2HCO-3
Hydrogen Sulfide can occur in the aqueous phase in two forms, as Hydrogen Sulfide or H2S and the bisulfide ion or HS-. The form that Hydrogen Sulfide will take is dependent on the pH of the liquid. Hydrogen Sulfide (H2S) and its conjugate base the bisulfide ion (HS), are referred to as total sulfide and occur together naturally at the pH ranges found in Florida ground water. As can be seen in the figure below, between the pH of 6 to 9, Hydrogen Sulfide can be present as Hydrogen Sulfide (H2S).
Hydrogen Sulfide is a very volatile dissolved gas and readily escapes as a gas into the air causing
unpleasant odors. At pH of 7 both forms of Sulfide are present in equal concentrations. Downwardly
adjusting pH to around 6 will result in converting all of the bisulfide ion, to the volatile form. Conversely,
raising the pH to above 9, will convert the bisulfide ion to the sulfide ion. These relationships are shown
in the figure below.
The four possible forms of Hydrogen Sulfide are shown in the figure above. At neutral pH, about half of the sulfide exists as hydrogen sulfide and the other half as HS-. Besides these four forms of hydrogen sulfide, other organic sulfur compounds, which are also odorous, can be formed through sulfate reduction initiated by decomposition of dissolved organic matter.
Enzymes increase biogas production by accelerating the breakdown of various materials such as maize silage, grasses, straw, as well as manures and some food wastes.
Using Enzymes to Enhance Biogas Production
Generally, polymeric carbohydrates, proteins and lipids present in waste water cannot be taken up by microscopic cells directly in the anaerobic process. Therefore, cellulose, amylase, protease, and lipase are little amounts of enzymes produced by microorganisms to breakdown and solubilize the macromolecular structures into soluble matter such as simple sugars, glycerol and fatty acids, amino acids to simplify transport through the cell film . Direct microbial enzymatic hydrolysis of lignocellulose produces less than 20% of glucose from cellulosic fraction and biogas production usually don’t exceed 60% of the theoretical value (475 L CH4/kgVS). Therefore, pretreatment of the substrate using enzymes is needed to make the holocelluloses more accessible to microbial attack and to improve hydrolysis.
Enzymes increase biogas production by accelerating the breakdown of various materials such as maize silage, grasses, straw, as well as manures and some food wastes, resulting in sugars more suitable for conversion into biogas. However, the enzymes have to be added continuously to the digester, are expensive to use and generally produce only 12-15% improvement in biogas production
Setzyme™ removes filamentous organisms. Sulfide toxicity reduces pH, thereby promoting the growth of fungi and lowers pH, which also promotes the growth of filamentous organisms.
Removing Filamentous Organisms
Activated sludge, the essential component of this process, includes a mixed and variable consortium of micro- and macro-organisms including viruses, bacteria, protozoa and metazoa who function in the transformation of organic materials into a liquid mixture that is relatively low in suspended solids and organic compounds. Among the key organic degraders, filamentous bacteria bond to other floc-forming organisms with biopolymers and provide important floc “backbones” to support the structure and shape of compact flocs for efficient sludge settling. The excessive growth of filamentous bacteria, however, trap and stabilize air bubbles when combined with biosurfactants, resulting in stable sludge foaming and biomass bulking on the surface of aerated reactors. Sludge bulking is a frequent operational problem and prevents adequate flocculation and impedes proper sludge mass settling.
Current control methods for sludge bulking depend on physio-chemical methods, including return sludge flow rate and aeration manipulation, surfactants and chlorine or hydrogen peroxide addition. Chlorine addition is especially an issue since it forms halocarbons, a highly carcinogenic, volatile chemical. Chlorine also increases the soluble BOD due to cell lysis and damages the active bacteria, needed for biological treatment of the wastewater.
One of the parameters used to define sludge bulking is Sludge Volume Index (SVI), which is defined as the volume of sludge (mL) which settles in 3o minutes using 1 liter of wastewater sample.
Sludge with very few filamentous organisms will have SVI less than 70 mL/g, while bulking sludge will have a SVI greater than 150 mL/g. Bulking sludge will have a clear effluent but when the wastewater leaves the clarifier, it will contain large pieces of sludge, which increases the total suspended solids (TSS) in the effluent.
Causes of sludge bulking are summarized in Table 1. The most common causes are low dissolved oxygen and low Food to Microorganism (F/M) ratio, due to starvation conditions in the influent wastewater.
ENZYMES TO REDUCE SLUDGE BULKING
Water Warriors SETZYME is an enzyme formulation which is designed to reduce filamentous microbe populations, improve sludge settling and enhance overall system performance, while reducing operating costs. The formulation contains a combination of enzymes and other ingredients selected to destabilize many filamentous microbial forms and thus improve settling.
COMPETITIVE STRATEGIES AND PRODUCTS
Competitive strategies includes the following: (1) adding biodegradable carbon to the influent to increase the feed to microorganism ratio; (2) using biocides such as chlorine or hydrogen peroxide to kill the filamentous organisms in the settlers; and (3) increase the amount of sludge being wasted in the activated sludge process. Biodegradable carbon, such as sugar, methanol, etc., provides more food to the microorganisms and increases the feed/microorganism ratio, which causes the growth of filamentous organisms. However, this also increases the treatment cost, since the added organic carbon has to be treated through the aeration, settling and sludge wasting process, which consumes operating cost.
Using biocides, such as chlorine or hydrogen peroxide to kill filamentous organisms is a common practice. While these biocides do kill the filamentous organisms, it also kills regular bacteria, which reduces the concentration of active bacteria in the process. Besides, these biocides are expensive to use, especially when the plant flowrates are large.
The third option is to reduce the amount of sludge recycle and waste more sludge. This reduces the bacteria concentration, which essentially increases the food to microorganism ratio. However, wasting more sludge also increases cost, since the wasted sludge has to be dewatered and landfilled.